diff --git a/middleware/generate.go b/middleware/generate.go
new file mode 100644
index 00000000..74e2d5eb
--- /dev/null
+++ b/middleware/generate.go
@@ -0,0 +1,530 @@
+//go:build ignore
+// +build ignore
+
+/*
+Usage: execute from repository root:
+$ go run middleware/generate.go && gofmt -w -s middleware/step_*.go
+*/
+
+/*
+This file produces the discrete middleware steps used by generated Smithy
+clients. Extreme care should be taken when editing the generation template.
+
+We are electing to generate these steps for performance reasons. A previous
+implementation of each step delegated to an underlying pair of list/mapping of
+IDs to middlewares. This enabled code reuse but was expensive at runtime,
+accounting for roughly 10% of the overall allocations in an operation.
+
+The generated step here instead operates by directly decorating the phase
+function in-place, which entirely removes that overhead and conveys a slight
+runtime performance boost.
+
+Hand-writing the decorator code in terms of generics is not feasible due to the
+excessive amount of adapter structures that need to be used.
+*/
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"strings"
+	"text/template"
+)
+
+const stepTemplate = `// Code generated by smithy-go/middleware/generate.go DO NOT EDIT.
+package middleware
+
+import (
+	"context"
+	"fmt"
+)
+
+{{ .InputDoc }}
+type {{.Phase}}Input struct {
+{{- range .Inputs }}
+	{{ .Name }} {{ .Type }}
+{{- end }}
+}
+
+// {{.Phase}}Output provides the result returned by the next {{.Phase}}Handler.
+type {{.Phase}}Output struct {
+{{- range .Outputs }}
+	{{ .Name }} {{ .Type }}
+{{- end }}
+}
+
+// {{.Phase}}Handler provides the interface for the next handler the
+// {{.Phase}}Middleware will call in the middleware chain.
+type {{.Phase}}Handler interface {
+	Handle{{.Phase}}(ctx context.Context, in {{.Phase}}Input) (
+		out {{.Phase}}Output, metadata Metadata, err error,
+	)
+}
+
+// {{.Phase}}Middleware provides the interface for middleware specific to the
+// {{.PhaseLowercase}} step. Delegates to the next {{.Phase}}Handler for further
+// processing.
+type {{.Phase}}Middleware interface {
+	// ID returns a unique ID for the middleware in the {{.Phase}}Step. The step does not
+	// allow duplicate IDs.
+	ID() string
+
+	// Handle{{.Phase}} invokes the middleware behavior which must delegate to the next handler
+	// for the middleware chain to continue. The method must return a result or
+	// error to its caller.
+	Handle{{.Phase}}(ctx context.Context, in {{.Phase}}Input, next {{.Phase}}Handler) (
+		out {{.Phase}}Output, metadata Metadata, err error,
+	)
+}
+
+// {{.Phase}}MiddlewareFunc returns a {{.Phase}}Middleware with the unique ID provided,
+// and the func to be invoked.
+func {{.Phase}}MiddlewareFunc(id string, fn func(context.Context, {{.Phase}}Input, {{.Phase}}Handler) ({{.Phase}}Output, Metadata, error)) {{.Phase}}Middleware {
+	return {{.PhaseLowercase}}MiddlewareFunc{
+		id: id,
+		fn: fn,
+	}
+}
+
+type {{.PhaseLowercase}}MiddlewareFunc struct {
+	// Unique ID for the middleware.
+	id string
+
+	// Middleware function to be called.
+	fn func(context.Context, {{.Phase}}Input, {{.Phase}}Handler) (
+		{{.Phase}}Output, Metadata, error,
+	)
+}
+
+// ID returns the unique ID for the middleware.
+func (s {{.PhaseLowercase}}MiddlewareFunc) ID() string { return s.id }
+
+// Handle{{.Phase}} invokes the middleware Fn.
+func (s {{.PhaseLowercase}}MiddlewareFunc) Handle{{.Phase}}(ctx context.Context, in {{.Phase}}Input, next {{.Phase}}Handler) (
+	out {{.Phase}}Output, metadata Metadata, err error,
+) {
+	return s.fn(ctx, in, next)
+}
+
+var _ {{.Phase}}Middleware = ({{.PhaseLowercase}}MiddlewareFunc{})
+
+// {{.Phase}}Step provides the ordered grouping of {{.Phase}}Middleware to be
+// invoked on a handler.
+type {{.Phase}}Step struct {
+	head *decorated{{.Phase}}Handler
+	tail *decorated{{.Phase}}Handler
+{{- range .StepFields }}
+	{{ .Name }} {{ .Type }}
+{{- end }}
+}
+
+// New{{.Phase}}Step returns an {{.Phase}}Step ready to have middleware for
+// {{.PhaseLowercase}} added to it.
+func New{{.Phase}}Step({{ range .StepFields }} {{ .Name }} {{ .Type }}, {{ end }}) *{{.Phase}}Step {
+	return &{{.Phase}}Step{
+{{- range .StepFields }}
+	{{ .Name }}: {{ .Name }},
+{{- end }}
+	}
+}
+
+var _ Middleware = (*{{.Phase}}Step)(nil)
+
+// ID returns the unique ID of the step as a middleware.
+func (s *{{.Phase}}Step) ID() string {
+	return "{{.Phase}} stack step"
+}
+
+// HandleMiddleware invokes the middleware by decorating the next handler
+// provided. Returns the result of the middleware and handler being invoked.
+//
+// Implements Middleware interface.
+func (s *{{.Phase}}Step) HandleMiddleware(ctx context.Context, in interface{}, next Handler) (
+	out interface{}, metadata Metadata, err error,
+) {
+	sIn := {{.Phase}}Input{
+{{- range .Inputs }}
+	{{ .Name }}: {{ .Binding }},
+{{- end }}
+	}
+
+	wh := &{{.PhaseLowercase}}WrapHandler{next}
+	if s.head == nil {
+		res, metadata, err := wh.Handle{{.Phase}}(ctx, sIn)
+		return res.Result, metadata, err
+	}
+
+	s.tail.Next = wh
+	res, metadata, err := s.head.Handle{{.Phase}}(ctx, sIn)
+	return res.Result, metadata, err
+}
+
+// Get retrieves the middleware identified by id. If the middleware is not present, returns false.
+func (s *{{.Phase}}Step) Get(id string) ({{.Phase}}Middleware, bool) {
+	for h := s.head; h != nil; {
+		if h.With.ID() == id {
+			return h.With, true
+		}
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decorated{{.Phase}}Handler)
+	}
+	return nil, false
+}
+
+// Add injects the middleware to the relative position of the middleware group.
+//
+// Add never returns an error. It used to for duplicate phases but this
+// behavior has since been removed as part of a performance optimization. The
+// return value from Add can be ignored.
+func (s *{{.Phase}}Step) Add(m {{.Phase}}Middleware, pos RelativePosition) error {
+	if s.head == nil {
+		s.head = &decorated{{.Phase}}Handler{nil, m}
+		s.tail = s.head
+		return nil
+	}
+
+	if pos == Before {
+		s.head = &decorated{{.Phase}}Handler{s.head, m}
+	} else {
+		tail := &decorated{{.Phase}}Handler{nil, m}
+		s.tail.Next = tail
+		s.tail = tail
+	}
+
+	return nil
+}
+
+// Insert injects the middleware relative to an existing middleware ID.
+// Returns error if the original middleware does not exist, or the middleware
+// being added already exists.
+func (s *{{.Phase}}Step) Insert(m {{.Phase}}Middleware, relativeTo string, pos RelativePosition) error {
+	var prev, found *decorated{{.Phase}}Handler
+	for h := s.head; h != nil; {
+		if h.With.ID() == relativeTo {
+			found = h
+			break
+		}
+		prev = h
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decorated{{.Phase}}Handler)
+	}
+	if found == nil {
+		return fmt.Errorf("not found: %s", m.ID())
+	}
+
+	if pos == Before {
+		if prev == nil { // at the front
+			s.head = &decorated{{.Phase}}Handler{s.head, m}
+		} else { // somewhere in the middle
+			prev.Next = &decorated{{.Phase}}Handler{found, m}
+		}
+	} else {
+		if found.Next == nil { // at the end
+			tail := &decorated{{.Phase}}Handler{nil, m}
+			s.tail.Next = tail
+			s.tail = tail
+		} else { // somewhere in the middle
+			found.Next = &decorated{{.Phase}}Handler{found.Next, m}
+		}
+	}
+
+	return nil
+}
+
+// Swap removes the middleware by id, replacing it with the new middleware.
+// Returns the middleware removed, or error if the middleware to be removed
+// doesn't exist.
+func (s *{{.Phase}}Step) Swap(id string, m {{.Phase}}Middleware) ({{.Phase}}Middleware, error) {
+	for h := s.head; h != nil; {
+		if h.With.ID() == id {
+			swapped := h.With
+			h.With = m
+			return swapped, nil
+		}
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decorated{{.Phase}}Handler)
+	}
+	return nil, fmt.Errorf("not found: %s", m.ID())
+}
+
+// Remove removes the middleware by id. Returns error if the middleware
+// doesn't exist.
+func (s *{{.Phase}}Step) Remove(id string) ({{.Phase}}Middleware, error) {
+	var prev, found *decorated{{.Phase}}Handler
+	for h := s.head; h != nil; {
+		if h.With.ID() == id {
+			found = h
+			break
+		}
+		prev = h
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decorated{{.Phase}}Handler)
+	}
+	if found == nil {
+		return nil, fmt.Errorf("not found: %s", id)
+	}
+
+	if s.head == s.tail { // it's the only one
+		s.head = nil
+		s.tail = nil
+	} else if found == s.head { // at the front
+		s.head = s.head.Next.(*decorated{{.Phase}}Handler)
+	} else if found == s.tail { // at the end
+		prev.Next = nil
+		s.tail = prev
+	} else {
+		prev.Next = found.Next // somewhere in the middle
+	}
+
+	return found.With, nil
+}
+
+// List returns a list of the middleware in the step.
+func (s *{{.Phase}}Step) List() []string {
+	var ids []string
+	for h := s.head; h != nil; {
+		ids = append(ids, h.With.ID())
+		if h.Next == nil {
+			break
+		}
+
+		// once executed, tail.Next of the list will be set to an
+		// *{{.PhaseLowercase}}WrapHandler, make sure to check for that
+		if hnext, ok := h.Next.(*decorated{{.Phase}}Handler); ok {
+			h = hnext
+		}
+	}
+	return ids
+}
+
+// Clear removes all middleware in the step.
+func (s *{{.Phase}}Step) Clear() {
+	s.head = nil
+	s.tail = nil
+}
+
+type {{.PhaseLowercase}}WrapHandler struct {
+	Next Handler
+}
+
+var _ {{.Phase}}Handler = (*{{.PhaseLowercase}}WrapHandler)(nil)
+
+// Handle{{.Phase}} implements {{.Phase}}Handler, converts types and delegates to underlying
+// generic handler.
+func (w {{.PhaseLowercase}}WrapHandler) Handle{{.Phase}}(ctx context.Context, in {{.Phase}}Input) (
+	out {{.Phase}}Output, metadata Metadata, err error,
+) {
+	res, metadata, err := w.Next.Handle(ctx, {{ .NextInput }})
+	return {{.Phase}}Output{
+{{- range .Outputs }}
+	{{ .Name }}: {{ .Binding }},
+{{- end }}
+	}, metadata, err
+}
+
+type decorated{{.Phase}}Handler struct {
+	Next {{.Phase}}Handler
+	With {{.Phase}}Middleware
+}
+
+var _ {{.Phase}}Handler = (*decorated{{.Phase}}Handler)(nil)
+
+func (h decorated{{.Phase}}Handler) Handle{{.Phase}}(ctx context.Context, in {{.Phase}}Input) (
+	out {{.Phase}}Output, metadata Metadata, err error,
+) {
+	return h.With.Handle{{.Phase}}(ctx, in, h.Next)
+}
+
+// {{.Phase}}HandlerFunc provides a wrapper around a function to be used as {{.PhaseLowercase}}Middleware.
+type {{.Phase}}HandlerFunc func(context.Context, {{.Phase}}Input) ({{.Phase}}Output, Metadata, error)
+
+// Handle{{.Phase}} calls the wrapped function with the provided arguments.
+func (f {{.Phase}}HandlerFunc) Handle{{.Phase}}(ctx context.Context, in {{.Phase}}Input) ({{.Phase}}Output, Metadata, error) {
+	return f(ctx, in)
+}
+
+var _ {{.Phase}}Handler = {{.Phase}}HandlerFunc(nil)
+`
+
+// Step describes how to generate a middleware.
+type Step struct {
+	Phase          string
+	PhaseLowercase string // set automatically by generator
+
+	// the input literal passed along to the phase that follows this one
+	//
+	// for the final phase that would just be the transport client call
+	// (so for HTTP, the request)
+	NextInput string
+
+	// struct fields on the step itself
+	StepFields []FieldDecl
+
+	InputDoc string
+	Inputs   []InputFieldDecl
+
+	Outputs []OutputFieldDecl
+}
+
+type FieldDecl struct {
+	Name, Type string
+}
+
+type InputFieldDecl struct {
+	FieldDecl
+	Binding string
+}
+
+type OutputFieldDecl struct {
+	FieldDecl
+	Binding string
+}
+
+func (s *Step) Filepath() string {
+	return fmt.Sprintf("middleware/step_%s.go", s.PhaseLowercase)
+}
+
+func main() {
+	tmpl, err := template.New("Step").Parse(stepTemplate)
+	if err != nil {
+		panic(err)
+	}
+
+	for _, step := range []Step{
+		{
+			Phase:     "Initialize",
+			NextInput: "in.Parameters",
+
+			InputDoc: `// InitializeInput wraps the input parameters for the InitializeMiddlewares to
+// consume. InitializeMiddleware may modify the parameter value before
+// forwarding it along to the next InitializeHandler.
+`,
+			Inputs: []InputFieldDecl{
+				{
+					FieldDecl: FieldDecl{"Parameters", "interface{}"},
+					Binding:   "in",
+				},
+			},
+			Outputs: []OutputFieldDecl{
+				{
+					FieldDecl: FieldDecl{"Result", "interface{}"},
+					Binding:   "res",
+				},
+			},
+		},
+		{
+			Phase:     "Serialize",
+			NextInput: "in.Request",
+			StepFields: []FieldDecl{
+				{"newRequest", "func() interface{}"},
+			},
+
+			InputDoc: `// SerializeInput provides the input parameters for the SerializeMiddleware to
+// consume. SerializeMiddleware may modify the Request value before forwarding
+// SerializeInput along to the next SerializeHandler. The Parameters member
+// should not be modified by SerializeMiddleware, InitializeMiddleware should
+// be responsible for modifying the provided Parameter value.`,
+			Inputs: []InputFieldDecl{
+				{
+					FieldDecl: FieldDecl{"Parameters", "interface{}"},
+					Binding:   "in",
+				},
+				{
+					FieldDecl: FieldDecl{"Request", "interface{}"},
+					Binding:   "s.newRequest()",
+				},
+			},
+			Outputs: []OutputFieldDecl{
+				{
+					FieldDecl: FieldDecl{"Result", "interface{}"},
+					Binding:   "res",
+				},
+			},
+		},
+		{
+			Phase:     "Build",
+			NextInput: "in.Request",
+
+			InputDoc: `// BuildInput provides the input parameters for the BuildMiddleware to consume.
+// BuildMiddleware may modify the Request value before forwarding the input
+// along to the next BuildHandler.`,
+			Inputs: []InputFieldDecl{
+				{
+					FieldDecl: FieldDecl{"Request", "interface{}"},
+					Binding:   "in",
+				},
+			},
+			Outputs: []OutputFieldDecl{
+				{
+					FieldDecl: FieldDecl{"Result", "interface{}"},
+					Binding:   "res",
+				},
+			},
+		},
+		{
+			Phase:     "Finalize",
+			NextInput: "in.Request",
+
+			InputDoc: `// FinalizeInput provides the input parameters for the FinalizeMiddleware to
+// consume. FinalizeMiddleware may modify the Request value before forwarding
+// the FinalizeInput along to the next next FinalizeHandler.`,
+			Inputs: []InputFieldDecl{
+				{
+					FieldDecl: FieldDecl{"Request", "interface{}"},
+					Binding:   "in",
+				},
+			},
+			Outputs: []OutputFieldDecl{
+				{
+					FieldDecl: FieldDecl{"Result", "interface{}"},
+					Binding:   "res",
+				},
+			},
+		},
+		{
+			Phase:     "Deserialize",
+			NextInput: "in.Request",
+
+			InputDoc: `// DeserializeInput provides the input parameters for the DeserializeInput to
+// consume. DeserializeMiddleware should not modify the Request, and instead
+// forward it along to the next DeserializeHandler.`,
+			Inputs: []InputFieldDecl{
+				{
+					FieldDecl: FieldDecl{"Request", "interface{}"},
+					Binding:   "in",
+				},
+			},
+			Outputs: []OutputFieldDecl{
+				{
+					FieldDecl: FieldDecl{"RawResponse", "interface{}"},
+					Binding:   "res",
+				},
+				{
+					FieldDecl: FieldDecl{"Result", "interface{}"},
+					Binding:   "nil",
+				},
+			},
+		},
+	} {
+		step.PhaseLowercase = strings.ToLower(step.Phase)
+
+		f, err := os.Create(step.Filepath())
+		if err != nil {
+			panic(err)
+		}
+		defer f.Close()
+
+		if err := tmpl.Execute(f, step); err != nil {
+			panic(err)
+		}
+	}
+}
diff --git a/middleware/step_build.go b/middleware/step_build.go
index cc7fe89c..d612c5e2 100644
--- a/middleware/step_build.go
+++ b/middleware/step_build.go
@@ -1,7 +1,9 @@
+// Code generated by smithy-go/middleware/generate.go DO NOT EDIT.
 package middleware
 
 import (
 	"context"
+	"fmt"
 )
 
 // BuildInput provides the input parameters for the BuildMiddleware to consume.
@@ -25,14 +27,14 @@ type BuildHandler interface {
 }
 
 // BuildMiddleware provides the interface for middleware specific to the
-// serialize step. Delegates to the next BuildHandler for further
+// build step. Delegates to the next BuildHandler for further
 // processing.
 type BuildMiddleware interface {
-	// Unique ID for the middleware in theBuildStep. The step does not allow
-	// duplicate IDs.
+	// ID returns a unique ID for the middleware in the BuildStep. The step does not
+	// allow duplicate IDs.
 	ID() string
 
-	// Invokes the middleware behavior which must delegate to the next handler
+	// HandleBuild invokes the middleware behavior which must delegate to the next handler
 	// for the middleware chain to continue. The method must return a result or
 	// error to its caller.
 	HandleBuild(ctx context.Context, in BuildInput, next BuildHandler) (
@@ -54,7 +56,9 @@ type buildMiddlewareFunc struct {
 	id string
 
 	// Middleware function to be called.
-	fn func(context.Context, BuildInput, BuildHandler) (BuildOutput, Metadata, error)
+	fn func(context.Context, BuildInput, BuildHandler) (
+		BuildOutput, Metadata, error,
+	)
 }
 
 // ID returns the unique ID for the middleware.
@@ -69,23 +73,22 @@ func (s buildMiddlewareFunc) HandleBuild(ctx context.Context, in BuildInput, nex
 
 var _ BuildMiddleware = (buildMiddlewareFunc{})
 
-// BuildStep provides the ordered grouping of BuildMiddleware to be invoked on
-// a handler.
+// BuildStep provides the ordered grouping of BuildMiddleware to be
+// invoked on a handler.
 type BuildStep struct {
-	ids *orderedIDs
+	head *decoratedBuildHandler
+	tail *decoratedBuildHandler
 }
 
-// NewBuildStep returns a BuildStep ready to have middleware for
-// initialization added to it.
+// NewBuildStep returns an BuildStep ready to have middleware for
+// build added to it.
 func NewBuildStep() *BuildStep {
-	return &BuildStep{
-		ids: newOrderedIDs(baseOrderedItems),
-	}
+	return &BuildStep{}
 }
 
 var _ Middleware = (*BuildStep)(nil)
 
-// ID returns the unique name of the step as a middleware.
+// ID returns the unique ID of the step as a middleware.
 func (s *BuildStep) ID() string {
 	return "Build stack step"
 }
@@ -97,77 +100,171 @@ func (s *BuildStep) ID() string {
 func (s *BuildStep) HandleMiddleware(ctx context.Context, in interface{}, next Handler) (
 	out interface{}, metadata Metadata, err error,
 ) {
-	order := s.ids.GetOrder()
-
-	var h BuildHandler = buildWrapHandler{Next: next}
-	for i := len(order) - 1; i >= 0; i-- {
-		h = decoratedBuildHandler{
-			Next: h,
-			With: order[i].(BuildMiddleware),
-		}
-	}
-
 	sIn := BuildInput{
 		Request: in,
 	}
 
-	res, metadata, err := h.HandleBuild(ctx, sIn)
+	wh := &buildWrapHandler{next}
+	if s.head == nil {
+		res, metadata, err := wh.HandleBuild(ctx, sIn)
+		return res.Result, metadata, err
+	}
+
+	s.tail.Next = wh
+	res, metadata, err := s.head.HandleBuild(ctx, sIn)
 	return res.Result, metadata, err
 }
 
 // Get retrieves the middleware identified by id. If the middleware is not present, returns false.
 func (s *BuildStep) Get(id string) (BuildMiddleware, bool) {
-	get, ok := s.ids.Get(id)
-	if !ok {
-		return nil, false
+	for h := s.head; h != nil; {
+		if h.With.ID() == id {
+			return h.With, true
+		}
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedBuildHandler)
 	}
-	return get.(BuildMiddleware), ok
+	return nil, false
 }
 
 // Add injects the middleware to the relative position of the middleware group.
-// Returns an error if the middleware already exists.
+//
+// Add never returns an error. It used to for duplicate phases but this
+// behavior has since been removed as part of a performance optimization. The
+// return value from Add can be ignored.
 func (s *BuildStep) Add(m BuildMiddleware, pos RelativePosition) error {
-	return s.ids.Add(m, pos)
+	if s.head == nil {
+		s.head = &decoratedBuildHandler{nil, m}
+		s.tail = s.head
+		return nil
+	}
+
+	if pos == Before {
+		s.head = &decoratedBuildHandler{s.head, m}
+	} else {
+		tail := &decoratedBuildHandler{nil, m}
+		s.tail.Next = tail
+		s.tail = tail
+	}
+
+	return nil
 }
 
-// Insert injects the middleware relative to an existing middleware id.
-// Returns an error if the original middleware does not exist, or the middleware
+// Insert injects the middleware relative to an existing middleware ID.
+// Returns error if the original middleware does not exist, or the middleware
 // being added already exists.
 func (s *BuildStep) Insert(m BuildMiddleware, relativeTo string, pos RelativePosition) error {
-	return s.ids.Insert(m, relativeTo, pos)
+	var prev, found *decoratedBuildHandler
+	for h := s.head; h != nil; {
+		if h.With.ID() == relativeTo {
+			found = h
+			break
+		}
+		prev = h
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedBuildHandler)
+	}
+	if found == nil {
+		return fmt.Errorf("not found: %s", m.ID())
+	}
+
+	if pos == Before {
+		if prev == nil { // at the front
+			s.head = &decoratedBuildHandler{s.head, m}
+		} else { // somewhere in the middle
+			prev.Next = &decoratedBuildHandler{found, m}
+		}
+	} else {
+		if found.Next == nil { // at the end
+			tail := &decoratedBuildHandler{nil, m}
+			s.tail.Next = tail
+			s.tail = tail
+		} else { // somewhere in the middle
+			found.Next = &decoratedBuildHandler{found.Next, m}
+		}
+	}
+
+	return nil
 }
 
 // Swap removes the middleware by id, replacing it with the new middleware.
-// Returns the middleware removed, or an error if the middleware to be removed
+// Returns the middleware removed, or error if the middleware to be removed
 // doesn't exist.
 func (s *BuildStep) Swap(id string, m BuildMiddleware) (BuildMiddleware, error) {
-	removed, err := s.ids.Swap(id, m)
-	if err != nil {
-		return nil, err
+	for h := s.head; h != nil; {
+		if h.With.ID() == id {
+			swapped := h.With
+			h.With = m
+			return swapped, nil
+		}
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedBuildHandler)
 	}
-
-	return removed.(BuildMiddleware), nil
+	return nil, fmt.Errorf("not found: %s", m.ID())
 }
 
 // Remove removes the middleware by id. Returns error if the middleware
 // doesn't exist.
 func (s *BuildStep) Remove(id string) (BuildMiddleware, error) {
-	removed, err := s.ids.Remove(id)
-	if err != nil {
-		return nil, err
+	var prev, found *decoratedBuildHandler
+	for h := s.head; h != nil; {
+		if h.With.ID() == id {
+			found = h
+			break
+		}
+		prev = h
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedBuildHandler)
+	}
+	if found == nil {
+		return nil, fmt.Errorf("not found: %s", id)
+	}
+
+	if s.head == s.tail { // it's the only one
+		s.head = nil
+		s.tail = nil
+	} else if found == s.head { // at the front
+		s.head = s.head.Next.(*decoratedBuildHandler)
+	} else if found == s.tail { // at the end
+		prev.Next = nil
+		s.tail = prev
+	} else {
+		prev.Next = found.Next // somewhere in the middle
 	}
 
-	return removed.(BuildMiddleware), nil
+	return found.With, nil
 }
 
 // List returns a list of the middleware in the step.
 func (s *BuildStep) List() []string {
-	return s.ids.List()
+	var ids []string
+	for h := s.head; h != nil; {
+		ids = append(ids, h.With.ID())
+		if h.Next == nil {
+			break
+		}
+
+		// once executed, tail.Next of the list will be set to an
+		// *buildWrapHandler, make sure to check for that
+		if hnext, ok := h.Next.(*decoratedBuildHandler); ok {
+			h = hnext
+		}
+	}
+	return ids
 }
 
 // Clear removes all middleware in the step.
 func (s *BuildStep) Clear() {
-	s.ids.Clear()
+	s.head = nil
+	s.tail = nil
 }
 
 type buildWrapHandler struct {
@@ -176,7 +273,7 @@ type buildWrapHandler struct {
 
 var _ BuildHandler = (*buildWrapHandler)(nil)
 
-// Implements BuildHandler, converts types and delegates to underlying
+// HandleBuild implements BuildHandler, converts types and delegates to underlying
 // generic handler.
 func (w buildWrapHandler) HandleBuild(ctx context.Context, in BuildInput) (
 	out BuildOutput, metadata Metadata, err error,
@@ -200,12 +297,12 @@ func (h decoratedBuildHandler) HandleBuild(ctx context.Context, in BuildInput) (
 	return h.With.HandleBuild(ctx, in, h.Next)
 }
 
-// BuildHandlerFunc provides a wrapper around a function to be used as a build middleware handler.
+// BuildHandlerFunc provides a wrapper around a function to be used as buildMiddleware.
 type BuildHandlerFunc func(context.Context, BuildInput) (BuildOutput, Metadata, error)
 
-// HandleBuild invokes the wrapped function with the provided arguments.
-func (b BuildHandlerFunc) HandleBuild(ctx context.Context, in BuildInput) (BuildOutput, Metadata, error) {
-	return b(ctx, in)
+// HandleBuild calls the wrapped function with the provided arguments.
+func (f BuildHandlerFunc) HandleBuild(ctx context.Context, in BuildInput) (BuildOutput, Metadata, error) {
+	return f(ctx, in)
 }
 
 var _ BuildHandler = BuildHandlerFunc(nil)
diff --git a/middleware/step_deserialize.go b/middleware/step_deserialize.go
index 9a6679a5..6485f30e 100644
--- a/middleware/step_deserialize.go
+++ b/middleware/step_deserialize.go
@@ -1,7 +1,9 @@
+// Code generated by smithy-go/middleware/generate.go DO NOT EDIT.
 package middleware
 
 import (
 	"context"
+	"fmt"
 )
 
 // DeserializeInput provides the input parameters for the DeserializeInput to
@@ -11,10 +13,7 @@ type DeserializeInput struct {
 	Request interface{}
 }
 
-// DeserializeOutput provides the result returned by the next
-// DeserializeHandler. The DeserializeMiddleware should deserialize the
-// RawResponse into a Result that can be consumed by middleware higher up in
-// the stack.
+// DeserializeOutput provides the result returned by the next DeserializeHandler.
 type DeserializeOutput struct {
 	RawResponse interface{}
 	Result      interface{}
@@ -29,7 +28,7 @@ type DeserializeHandler interface {
 }
 
 // DeserializeMiddleware provides the interface for middleware specific to the
-// serialize step. Delegates to the next DeserializeHandler for further
+// deserialize step. Delegates to the next DeserializeHandler for further
 // processing.
 type DeserializeMiddleware interface {
 	// ID returns a unique ID for the middleware in the DeserializeStep. The step does not
@@ -44,8 +43,8 @@ type DeserializeMiddleware interface {
 	)
 }
 
-// DeserializeMiddlewareFunc returns a DeserializeMiddleware with the unique ID
-// provided, and the func to be invoked.
+// DeserializeMiddlewareFunc returns a DeserializeMiddleware with the unique ID provided,
+// and the func to be invoked.
 func DeserializeMiddlewareFunc(id string, fn func(context.Context, DeserializeInput, DeserializeHandler) (DeserializeOutput, Metadata, error)) DeserializeMiddleware {
 	return deserializeMiddlewareFunc{
 		id: id,
@@ -78,16 +77,14 @@ var _ DeserializeMiddleware = (deserializeMiddlewareFunc{})
 // DeserializeStep provides the ordered grouping of DeserializeMiddleware to be
 // invoked on a handler.
 type DeserializeStep struct {
-	ids *orderedIDs
+	head *decoratedDeserializeHandler
+	tail *decoratedDeserializeHandler
 }
 
-// NewDeserializeStep returns a DeserializeStep ready to have middleware for
-// initialization added to it.
+// NewDeserializeStep returns an DeserializeStep ready to have middleware for
+// deserialize added to it.
 func NewDeserializeStep() *DeserializeStep {
-	return &DeserializeStep{
-		// downstream SDK typically has larger Deserialize step
-		ids: newOrderedIDs(baseOrderedItems * 2),
-	}
+	return &DeserializeStep{}
 }
 
 var _ Middleware = (*DeserializeStep)(nil)
@@ -104,77 +101,171 @@ func (s *DeserializeStep) ID() string {
 func (s *DeserializeStep) HandleMiddleware(ctx context.Context, in interface{}, next Handler) (
 	out interface{}, metadata Metadata, err error,
 ) {
-	order := s.ids.GetOrder()
-
-	var h DeserializeHandler = deserializeWrapHandler{Next: next}
-	for i := len(order) - 1; i >= 0; i-- {
-		h = decoratedDeserializeHandler{
-			Next: h,
-			With: order[i].(DeserializeMiddleware),
-		}
-	}
-
 	sIn := DeserializeInput{
 		Request: in,
 	}
 
-	res, metadata, err := h.HandleDeserialize(ctx, sIn)
+	wh := &deserializeWrapHandler{next}
+	if s.head == nil {
+		res, metadata, err := wh.HandleDeserialize(ctx, sIn)
+		return res.Result, metadata, err
+	}
+
+	s.tail.Next = wh
+	res, metadata, err := s.head.HandleDeserialize(ctx, sIn)
 	return res.Result, metadata, err
 }
 
 // Get retrieves the middleware identified by id. If the middleware is not present, returns false.
 func (s *DeserializeStep) Get(id string) (DeserializeMiddleware, bool) {
-	get, ok := s.ids.Get(id)
-	if !ok {
-		return nil, false
+	for h := s.head; h != nil; {
+		if h.With.ID() == id {
+			return h.With, true
+		}
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedDeserializeHandler)
 	}
-	return get.(DeserializeMiddleware), ok
+	return nil, false
 }
 
 // Add injects the middleware to the relative position of the middleware group.
-// Returns an error if the middleware already exists.
+//
+// Add never returns an error. It used to for duplicate phases but this
+// behavior has since been removed as part of a performance optimization. The
+// return value from Add can be ignored.
 func (s *DeserializeStep) Add(m DeserializeMiddleware, pos RelativePosition) error {
-	return s.ids.Add(m, pos)
+	if s.head == nil {
+		s.head = &decoratedDeserializeHandler{nil, m}
+		s.tail = s.head
+		return nil
+	}
+
+	if pos == Before {
+		s.head = &decoratedDeserializeHandler{s.head, m}
+	} else {
+		tail := &decoratedDeserializeHandler{nil, m}
+		s.tail.Next = tail
+		s.tail = tail
+	}
+
+	return nil
 }
 
 // Insert injects the middleware relative to an existing middleware ID.
 // Returns error if the original middleware does not exist, or the middleware
 // being added already exists.
 func (s *DeserializeStep) Insert(m DeserializeMiddleware, relativeTo string, pos RelativePosition) error {
-	return s.ids.Insert(m, relativeTo, pos)
+	var prev, found *decoratedDeserializeHandler
+	for h := s.head; h != nil; {
+		if h.With.ID() == relativeTo {
+			found = h
+			break
+		}
+		prev = h
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedDeserializeHandler)
+	}
+	if found == nil {
+		return fmt.Errorf("not found: %s", m.ID())
+	}
+
+	if pos == Before {
+		if prev == nil { // at the front
+			s.head = &decoratedDeserializeHandler{s.head, m}
+		} else { // somewhere in the middle
+			prev.Next = &decoratedDeserializeHandler{found, m}
+		}
+	} else {
+		if found.Next == nil { // at the end
+			tail := &decoratedDeserializeHandler{nil, m}
+			s.tail.Next = tail
+			s.tail = tail
+		} else { // somewhere in the middle
+			found.Next = &decoratedDeserializeHandler{found.Next, m}
+		}
+	}
+
+	return nil
 }
 
 // Swap removes the middleware by id, replacing it with the new middleware.
 // Returns the middleware removed, or error if the middleware to be removed
 // doesn't exist.
 func (s *DeserializeStep) Swap(id string, m DeserializeMiddleware) (DeserializeMiddleware, error) {
-	removed, err := s.ids.Swap(id, m)
-	if err != nil {
-		return nil, err
+	for h := s.head; h != nil; {
+		if h.With.ID() == id {
+			swapped := h.With
+			h.With = m
+			return swapped, nil
+		}
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedDeserializeHandler)
 	}
-
-	return removed.(DeserializeMiddleware), nil
+	return nil, fmt.Errorf("not found: %s", m.ID())
 }
 
 // Remove removes the middleware by id. Returns error if the middleware
 // doesn't exist.
 func (s *DeserializeStep) Remove(id string) (DeserializeMiddleware, error) {
-	removed, err := s.ids.Remove(id)
-	if err != nil {
-		return nil, err
+	var prev, found *decoratedDeserializeHandler
+	for h := s.head; h != nil; {
+		if h.With.ID() == id {
+			found = h
+			break
+		}
+		prev = h
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedDeserializeHandler)
+	}
+	if found == nil {
+		return nil, fmt.Errorf("not found: %s", id)
+	}
+
+	if s.head == s.tail { // it's the only one
+		s.head = nil
+		s.tail = nil
+	} else if found == s.head { // at the front
+		s.head = s.head.Next.(*decoratedDeserializeHandler)
+	} else if found == s.tail { // at the end
+		prev.Next = nil
+		s.tail = prev
+	} else {
+		prev.Next = found.Next // somewhere in the middle
 	}
 
-	return removed.(DeserializeMiddleware), nil
+	return found.With, nil
 }
 
 // List returns a list of the middleware in the step.
 func (s *DeserializeStep) List() []string {
-	return s.ids.List()
+	var ids []string
+	for h := s.head; h != nil; {
+		ids = append(ids, h.With.ID())
+		if h.Next == nil {
+			break
+		}
+
+		// once executed, tail.Next of the list will be set to an
+		// *deserializeWrapHandler, make sure to check for that
+		if hnext, ok := h.Next.(*decoratedDeserializeHandler); ok {
+			h = hnext
+		}
+	}
+	return ids
 }
 
 // Clear removes all middleware in the step.
 func (s *DeserializeStep) Clear() {
-	s.ids.Clear()
+	s.head = nil
+	s.tail = nil
 }
 
 type deserializeWrapHandler struct {
@@ -188,9 +279,10 @@ var _ DeserializeHandler = (*deserializeWrapHandler)(nil)
 func (w deserializeWrapHandler) HandleDeserialize(ctx context.Context, in DeserializeInput) (
 	out DeserializeOutput, metadata Metadata, err error,
 ) {
-	resp, metadata, err := w.Next.Handle(ctx, in.Request)
+	res, metadata, err := w.Next.Handle(ctx, in.Request)
 	return DeserializeOutput{
-		RawResponse: resp,
+		RawResponse: res,
+		Result:      nil,
 	}, metadata, err
 }
 
@@ -207,12 +299,12 @@ func (h decoratedDeserializeHandler) HandleDeserialize(ctx context.Context, in D
 	return h.With.HandleDeserialize(ctx, in, h.Next)
 }
 
-// DeserializeHandlerFunc provides a wrapper around a function to be used as a deserialize middleware handler.
+// DeserializeHandlerFunc provides a wrapper around a function to be used as deserializeMiddleware.
 type DeserializeHandlerFunc func(context.Context, DeserializeInput) (DeserializeOutput, Metadata, error)
 
-// HandleDeserialize invokes the wrapped function with the given arguments.
-func (d DeserializeHandlerFunc) HandleDeserialize(ctx context.Context, in DeserializeInput) (DeserializeOutput, Metadata, error) {
-	return d(ctx, in)
+// HandleDeserialize calls the wrapped function with the provided arguments.
+func (f DeserializeHandlerFunc) HandleDeserialize(ctx context.Context, in DeserializeInput) (DeserializeOutput, Metadata, error) {
+	return f(ctx, in)
 }
 
 var _ DeserializeHandler = DeserializeHandlerFunc(nil)
diff --git a/middleware/step_finalize.go b/middleware/step_finalize.go
index 76eab249..94b3425c 100644
--- a/middleware/step_finalize.go
+++ b/middleware/step_finalize.go
@@ -1,6 +1,10 @@
+// Code generated by smithy-go/middleware/generate.go DO NOT EDIT.
 package middleware
 
-import "context"
+import (
+	"context"
+	"fmt"
+)
 
 // FinalizeInput provides the input parameters for the FinalizeMiddleware to
 // consume. FinalizeMiddleware may modify the Request value before forwarding
@@ -23,7 +27,7 @@ type FinalizeHandler interface {
 }
 
 // FinalizeMiddleware provides the interface for middleware specific to the
-// serialize step. Delegates to the next FinalizeHandler for further
+// finalize step. Delegates to the next FinalizeHandler for further
 // processing.
 type FinalizeMiddleware interface {
 	// ID returns a unique ID for the middleware in the FinalizeStep. The step does not
@@ -38,8 +42,8 @@ type FinalizeMiddleware interface {
 	)
 }
 
-// FinalizeMiddlewareFunc returns a FinalizeMiddleware with the unique ID
-// provided, and the func to be invoked.
+// FinalizeMiddlewareFunc returns a FinalizeMiddleware with the unique ID provided,
+// and the func to be invoked.
 func FinalizeMiddlewareFunc(id string, fn func(context.Context, FinalizeInput, FinalizeHandler) (FinalizeOutput, Metadata, error)) FinalizeMiddleware {
 	return finalizeMiddlewareFunc{
 		id: id,
@@ -72,21 +76,19 @@ var _ FinalizeMiddleware = (finalizeMiddlewareFunc{})
 // FinalizeStep provides the ordered grouping of FinalizeMiddleware to be
 // invoked on a handler.
 type FinalizeStep struct {
-	ids *orderedIDs
+	head *decoratedFinalizeHandler
+	tail *decoratedFinalizeHandler
 }
 
-// NewFinalizeStep returns a FinalizeStep ready to have middleware for
-// initialization added to it.
+// NewFinalizeStep returns an FinalizeStep ready to have middleware for
+// finalize added to it.
 func NewFinalizeStep() *FinalizeStep {
-	return &FinalizeStep{
-		// downstream SDK typically has larger Finalize step
-		ids: newOrderedIDs(baseOrderedItems * 2),
-	}
+	return &FinalizeStep{}
 }
 
 var _ Middleware = (*FinalizeStep)(nil)
 
-// ID returns the unique id of the step as a middleware.
+// ID returns the unique ID of the step as a middleware.
 func (s *FinalizeStep) ID() string {
 	return "Finalize stack step"
 }
@@ -98,77 +100,171 @@ func (s *FinalizeStep) ID() string {
 func (s *FinalizeStep) HandleMiddleware(ctx context.Context, in interface{}, next Handler) (
 	out interface{}, metadata Metadata, err error,
 ) {
-	order := s.ids.GetOrder()
-
-	var h FinalizeHandler = finalizeWrapHandler{Next: next}
-	for i := len(order) - 1; i >= 0; i-- {
-		h = decoratedFinalizeHandler{
-			Next: h,
-			With: order[i].(FinalizeMiddleware),
-		}
-	}
-
 	sIn := FinalizeInput{
 		Request: in,
 	}
 
-	res, metadata, err := h.HandleFinalize(ctx, sIn)
+	wh := &finalizeWrapHandler{next}
+	if s.head == nil {
+		res, metadata, err := wh.HandleFinalize(ctx, sIn)
+		return res.Result, metadata, err
+	}
+
+	s.tail.Next = wh
+	res, metadata, err := s.head.HandleFinalize(ctx, sIn)
 	return res.Result, metadata, err
 }
 
 // Get retrieves the middleware identified by id. If the middleware is not present, returns false.
 func (s *FinalizeStep) Get(id string) (FinalizeMiddleware, bool) {
-	get, ok := s.ids.Get(id)
-	if !ok {
-		return nil, false
+	for h := s.head; h != nil; {
+		if h.With.ID() == id {
+			return h.With, true
+		}
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedFinalizeHandler)
 	}
-	return get.(FinalizeMiddleware), ok
+	return nil, false
 }
 
 // Add injects the middleware to the relative position of the middleware group.
-// Returns an error if the middleware already exists.
+//
+// Add never returns an error. It used to for duplicate phases but this
+// behavior has since been removed as part of a performance optimization. The
+// return value from Add can be ignored.
 func (s *FinalizeStep) Add(m FinalizeMiddleware, pos RelativePosition) error {
-	return s.ids.Add(m, pos)
+	if s.head == nil {
+		s.head = &decoratedFinalizeHandler{nil, m}
+		s.tail = s.head
+		return nil
+	}
+
+	if pos == Before {
+		s.head = &decoratedFinalizeHandler{s.head, m}
+	} else {
+		tail := &decoratedFinalizeHandler{nil, m}
+		s.tail.Next = tail
+		s.tail = tail
+	}
+
+	return nil
 }
 
 // Insert injects the middleware relative to an existing middleware ID.
 // Returns error if the original middleware does not exist, or the middleware
 // being added already exists.
 func (s *FinalizeStep) Insert(m FinalizeMiddleware, relativeTo string, pos RelativePosition) error {
-	return s.ids.Insert(m, relativeTo, pos)
+	var prev, found *decoratedFinalizeHandler
+	for h := s.head; h != nil; {
+		if h.With.ID() == relativeTo {
+			found = h
+			break
+		}
+		prev = h
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedFinalizeHandler)
+	}
+	if found == nil {
+		return fmt.Errorf("not found: %s", m.ID())
+	}
+
+	if pos == Before {
+		if prev == nil { // at the front
+			s.head = &decoratedFinalizeHandler{s.head, m}
+		} else { // somewhere in the middle
+			prev.Next = &decoratedFinalizeHandler{found, m}
+		}
+	} else {
+		if found.Next == nil { // at the end
+			tail := &decoratedFinalizeHandler{nil, m}
+			s.tail.Next = tail
+			s.tail = tail
+		} else { // somewhere in the middle
+			found.Next = &decoratedFinalizeHandler{found.Next, m}
+		}
+	}
+
+	return nil
 }
 
 // Swap removes the middleware by id, replacing it with the new middleware.
 // Returns the middleware removed, or error if the middleware to be removed
 // doesn't exist.
 func (s *FinalizeStep) Swap(id string, m FinalizeMiddleware) (FinalizeMiddleware, error) {
-	removed, err := s.ids.Swap(id, m)
-	if err != nil {
-		return nil, err
+	for h := s.head; h != nil; {
+		if h.With.ID() == id {
+			swapped := h.With
+			h.With = m
+			return swapped, nil
+		}
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedFinalizeHandler)
 	}
-
-	return removed.(FinalizeMiddleware), nil
+	return nil, fmt.Errorf("not found: %s", m.ID())
 }
 
 // Remove removes the middleware by id. Returns error if the middleware
 // doesn't exist.
 func (s *FinalizeStep) Remove(id string) (FinalizeMiddleware, error) {
-	removed, err := s.ids.Remove(id)
-	if err != nil {
-		return nil, err
+	var prev, found *decoratedFinalizeHandler
+	for h := s.head; h != nil; {
+		if h.With.ID() == id {
+			found = h
+			break
+		}
+		prev = h
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedFinalizeHandler)
+	}
+	if found == nil {
+		return nil, fmt.Errorf("not found: %s", id)
+	}
+
+	if s.head == s.tail { // it's the only one
+		s.head = nil
+		s.tail = nil
+	} else if found == s.head { // at the front
+		s.head = s.head.Next.(*decoratedFinalizeHandler)
+	} else if found == s.tail { // at the end
+		prev.Next = nil
+		s.tail = prev
+	} else {
+		prev.Next = found.Next // somewhere in the middle
 	}
 
-	return removed.(FinalizeMiddleware), nil
+	return found.With, nil
 }
 
 // List returns a list of the middleware in the step.
 func (s *FinalizeStep) List() []string {
-	return s.ids.List()
+	var ids []string
+	for h := s.head; h != nil; {
+		ids = append(ids, h.With.ID())
+		if h.Next == nil {
+			break
+		}
+
+		// once executed, tail.Next of the list will be set to an
+		// *finalizeWrapHandler, make sure to check for that
+		if hnext, ok := h.Next.(*decoratedFinalizeHandler); ok {
+			h = hnext
+		}
+	}
+	return ids
 }
 
 // Clear removes all middleware in the step.
 func (s *FinalizeStep) Clear() {
-	s.ids.Clear()
+	s.head = nil
+	s.tail = nil
 }
 
 type finalizeWrapHandler struct {
@@ -201,10 +297,10 @@ func (h decoratedFinalizeHandler) HandleFinalize(ctx context.Context, in Finaliz
 	return h.With.HandleFinalize(ctx, in, h.Next)
 }
 
-// FinalizeHandlerFunc provides a wrapper around a function to be used as a finalize middleware handler.
+// FinalizeHandlerFunc provides a wrapper around a function to be used as finalizeMiddleware.
 type FinalizeHandlerFunc func(context.Context, FinalizeInput) (FinalizeOutput, Metadata, error)
 
-// HandleFinalize invokes the wrapped function with the given arguments.
+// HandleFinalize calls the wrapped function with the provided arguments.
 func (f FinalizeHandlerFunc) HandleFinalize(ctx context.Context, in FinalizeInput) (FinalizeOutput, Metadata, error) {
 	return f(ctx, in)
 }
diff --git a/middleware/step_initialize.go b/middleware/step_initialize.go
index 312be3a3..91bf0ad1 100644
--- a/middleware/step_initialize.go
+++ b/middleware/step_initialize.go
@@ -1,10 +1,15 @@
+// Code generated by smithy-go/middleware/generate.go DO NOT EDIT.
 package middleware
 
-import "context"
+import (
+	"context"
+	"fmt"
+)
 
 // InitializeInput wraps the input parameters for the InitializeMiddlewares to
 // consume. InitializeMiddleware may modify the parameter value before
 // forwarding it along to the next InitializeHandler.
+
 type InitializeInput struct {
 	Parameters interface{}
 }
@@ -72,15 +77,14 @@ var _ InitializeMiddleware = (initializeMiddlewareFunc{})
 // InitializeStep provides the ordered grouping of InitializeMiddleware to be
 // invoked on a handler.
 type InitializeStep struct {
-	ids *orderedIDs
+	head *decoratedInitializeHandler
+	tail *decoratedInitializeHandler
 }
 
 // NewInitializeStep returns an InitializeStep ready to have middleware for
-// initialization added to it.
+// initialize added to it.
 func NewInitializeStep() *InitializeStep {
-	return &InitializeStep{
-		ids: newOrderedIDs(baseOrderedItems),
-	}
+	return &InitializeStep{}
 }
 
 var _ Middleware = (*InitializeStep)(nil)
@@ -97,77 +101,171 @@ func (s *InitializeStep) ID() string {
 func (s *InitializeStep) HandleMiddleware(ctx context.Context, in interface{}, next Handler) (
 	out interface{}, metadata Metadata, err error,
 ) {
-	order := s.ids.GetOrder()
-
-	var h InitializeHandler = initializeWrapHandler{Next: next}
-	for i := len(order) - 1; i >= 0; i-- {
-		h = decoratedInitializeHandler{
-			Next: h,
-			With: order[i].(InitializeMiddleware),
-		}
-	}
-
 	sIn := InitializeInput{
 		Parameters: in,
 	}
 
-	res, metadata, err := h.HandleInitialize(ctx, sIn)
+	wh := &initializeWrapHandler{next}
+	if s.head == nil {
+		res, metadata, err := wh.HandleInitialize(ctx, sIn)
+		return res.Result, metadata, err
+	}
+
+	s.tail.Next = wh
+	res, metadata, err := s.head.HandleInitialize(ctx, sIn)
 	return res.Result, metadata, err
 }
 
 // Get retrieves the middleware identified by id. If the middleware is not present, returns false.
 func (s *InitializeStep) Get(id string) (InitializeMiddleware, bool) {
-	get, ok := s.ids.Get(id)
-	if !ok {
-		return nil, false
+	for h := s.head; h != nil; {
+		if h.With.ID() == id {
+			return h.With, true
+		}
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedInitializeHandler)
 	}
-	return get.(InitializeMiddleware), ok
+	return nil, false
 }
 
 // Add injects the middleware to the relative position of the middleware group.
-// Returns an error if the middleware already exists.
+//
+// Add never returns an error. It used to for duplicate phases but this
+// behavior has since been removed as part of a performance optimization. The
+// return value from Add can be ignored.
 func (s *InitializeStep) Add(m InitializeMiddleware, pos RelativePosition) error {
-	return s.ids.Add(m, pos)
+	if s.head == nil {
+		s.head = &decoratedInitializeHandler{nil, m}
+		s.tail = s.head
+		return nil
+	}
+
+	if pos == Before {
+		s.head = &decoratedInitializeHandler{s.head, m}
+	} else {
+		tail := &decoratedInitializeHandler{nil, m}
+		s.tail.Next = tail
+		s.tail = tail
+	}
+
+	return nil
 }
 
 // Insert injects the middleware relative to an existing middleware ID.
 // Returns error if the original middleware does not exist, or the middleware
 // being added already exists.
 func (s *InitializeStep) Insert(m InitializeMiddleware, relativeTo string, pos RelativePosition) error {
-	return s.ids.Insert(m, relativeTo, pos)
+	var prev, found *decoratedInitializeHandler
+	for h := s.head; h != nil; {
+		if h.With.ID() == relativeTo {
+			found = h
+			break
+		}
+		prev = h
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedInitializeHandler)
+	}
+	if found == nil {
+		return fmt.Errorf("not found: %s", m.ID())
+	}
+
+	if pos == Before {
+		if prev == nil { // at the front
+			s.head = &decoratedInitializeHandler{s.head, m}
+		} else { // somewhere in the middle
+			prev.Next = &decoratedInitializeHandler{found, m}
+		}
+	} else {
+		if found.Next == nil { // at the end
+			tail := &decoratedInitializeHandler{nil, m}
+			s.tail.Next = tail
+			s.tail = tail
+		} else { // somewhere in the middle
+			found.Next = &decoratedInitializeHandler{found.Next, m}
+		}
+	}
+
+	return nil
 }
 
 // Swap removes the middleware by id, replacing it with the new middleware.
 // Returns the middleware removed, or error if the middleware to be removed
 // doesn't exist.
 func (s *InitializeStep) Swap(id string, m InitializeMiddleware) (InitializeMiddleware, error) {
-	removed, err := s.ids.Swap(id, m)
-	if err != nil {
-		return nil, err
+	for h := s.head; h != nil; {
+		if h.With.ID() == id {
+			swapped := h.With
+			h.With = m
+			return swapped, nil
+		}
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedInitializeHandler)
 	}
-
-	return removed.(InitializeMiddleware), nil
+	return nil, fmt.Errorf("not found: %s", m.ID())
 }
 
 // Remove removes the middleware by id. Returns error if the middleware
 // doesn't exist.
 func (s *InitializeStep) Remove(id string) (InitializeMiddleware, error) {
-	removed, err := s.ids.Remove(id)
-	if err != nil {
-		return nil, err
+	var prev, found *decoratedInitializeHandler
+	for h := s.head; h != nil; {
+		if h.With.ID() == id {
+			found = h
+			break
+		}
+		prev = h
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedInitializeHandler)
+	}
+	if found == nil {
+		return nil, fmt.Errorf("not found: %s", id)
+	}
+
+	if s.head == s.tail { // it's the only one
+		s.head = nil
+		s.tail = nil
+	} else if found == s.head { // at the front
+		s.head = s.head.Next.(*decoratedInitializeHandler)
+	} else if found == s.tail { // at the end
+		prev.Next = nil
+		s.tail = prev
+	} else {
+		prev.Next = found.Next // somewhere in the middle
 	}
 
-	return removed.(InitializeMiddleware), nil
+	return found.With, nil
 }
 
 // List returns a list of the middleware in the step.
 func (s *InitializeStep) List() []string {
-	return s.ids.List()
+	var ids []string
+	for h := s.head; h != nil; {
+		ids = append(ids, h.With.ID())
+		if h.Next == nil {
+			break
+		}
+
+		// once executed, tail.Next of the list will be set to an
+		// *initializeWrapHandler, make sure to check for that
+		if hnext, ok := h.Next.(*decoratedInitializeHandler); ok {
+			h = hnext
+		}
+	}
+	return ids
 }
 
 // Clear removes all middleware in the step.
 func (s *InitializeStep) Clear() {
-	s.ids.Clear()
+	s.head = nil
+	s.tail = nil
 }
 
 type initializeWrapHandler struct {
@@ -200,12 +298,12 @@ func (h decoratedInitializeHandler) HandleInitialize(ctx context.Context, in Ini
 	return h.With.HandleInitialize(ctx, in, h.Next)
 }
 
-// InitializeHandlerFunc provides a wrapper around a function to be used as an initialize middleware handler.
+// InitializeHandlerFunc provides a wrapper around a function to be used as initializeMiddleware.
 type InitializeHandlerFunc func(context.Context, InitializeInput) (InitializeOutput, Metadata, error)
 
 // HandleInitialize calls the wrapped function with the provided arguments.
-func (i InitializeHandlerFunc) HandleInitialize(ctx context.Context, in InitializeInput) (InitializeOutput, Metadata, error) {
-	return i(ctx, in)
+func (f InitializeHandlerFunc) HandleInitialize(ctx context.Context, in InitializeInput) (InitializeOutput, Metadata, error) {
+	return f(ctx, in)
 }
 
 var _ InitializeHandler = InitializeHandlerFunc(nil)
diff --git a/middleware/step_serialize.go b/middleware/step_serialize.go
index a4ce4bee..3b639bf8 100644
--- a/middleware/step_serialize.go
+++ b/middleware/step_serialize.go
@@ -1,6 +1,10 @@
+// Code generated by smithy-go/middleware/generate.go DO NOT EDIT.
 package middleware
 
-import "context"
+import (
+	"context"
+	"fmt"
+)
 
 // SerializeInput provides the input parameters for the SerializeMiddleware to
 // consume. SerializeMiddleware may modify the Request value before forwarding
@@ -41,8 +45,8 @@ type SerializeMiddleware interface {
 	)
 }
 
-// SerializeMiddlewareFunc returns a SerializeMiddleware with the unique ID
-// provided, and the func to be invoked.
+// SerializeMiddlewareFunc returns a SerializeMiddleware with the unique ID provided,
+// and the func to be invoked.
 func SerializeMiddlewareFunc(id string, fn func(context.Context, SerializeInput, SerializeHandler) (SerializeOutput, Metadata, error)) SerializeMiddleware {
 	return serializeMiddlewareFunc{
 		id: id,
@@ -75,17 +79,15 @@ var _ SerializeMiddleware = (serializeMiddlewareFunc{})
 // SerializeStep provides the ordered grouping of SerializeMiddleware to be
 // invoked on a handler.
 type SerializeStep struct {
+	head       *decoratedSerializeHandler
+	tail       *decoratedSerializeHandler
 	newRequest func() interface{}
-	ids        *orderedIDs
 }
 
-// NewSerializeStep returns a SerializeStep ready to have middleware for
-// initialization added to it. The newRequest func parameter is used to
-// initialize the transport specific request for the stack SerializeStep to
-// serialize the input parameters into.
+// NewSerializeStep returns an SerializeStep ready to have middleware for
+// serialize added to it.
 func NewSerializeStep(newRequest func() interface{}) *SerializeStep {
 	return &SerializeStep{
-		ids:        newOrderedIDs(baseOrderedItems),
 		newRequest: newRequest,
 	}
 }
@@ -104,78 +106,172 @@ func (s *SerializeStep) ID() string {
 func (s *SerializeStep) HandleMiddleware(ctx context.Context, in interface{}, next Handler) (
 	out interface{}, metadata Metadata, err error,
 ) {
-	order := s.ids.GetOrder()
-
-	var h SerializeHandler = serializeWrapHandler{Next: next}
-	for i := len(order) - 1; i >= 0; i-- {
-		h = decoratedSerializeHandler{
-			Next: h,
-			With: order[i].(SerializeMiddleware),
-		}
-	}
-
 	sIn := SerializeInput{
 		Parameters: in,
 		Request:    s.newRequest(),
 	}
 
-	res, metadata, err := h.HandleSerialize(ctx, sIn)
+	wh := &serializeWrapHandler{next}
+	if s.head == nil {
+		res, metadata, err := wh.HandleSerialize(ctx, sIn)
+		return res.Result, metadata, err
+	}
+
+	s.tail.Next = wh
+	res, metadata, err := s.head.HandleSerialize(ctx, sIn)
 	return res.Result, metadata, err
 }
 
 // Get retrieves the middleware identified by id. If the middleware is not present, returns false.
 func (s *SerializeStep) Get(id string) (SerializeMiddleware, bool) {
-	get, ok := s.ids.Get(id)
-	if !ok {
-		return nil, false
+	for h := s.head; h != nil; {
+		if h.With.ID() == id {
+			return h.With, true
+		}
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedSerializeHandler)
 	}
-	return get.(SerializeMiddleware), ok
+	return nil, false
 }
 
 // Add injects the middleware to the relative position of the middleware group.
-// Returns an error if the middleware already exists.
+//
+// Add never returns an error. It used to for duplicate phases but this
+// behavior has since been removed as part of a performance optimization. The
+// return value from Add can be ignored.
 func (s *SerializeStep) Add(m SerializeMiddleware, pos RelativePosition) error {
-	return s.ids.Add(m, pos)
+	if s.head == nil {
+		s.head = &decoratedSerializeHandler{nil, m}
+		s.tail = s.head
+		return nil
+	}
+
+	if pos == Before {
+		s.head = &decoratedSerializeHandler{s.head, m}
+	} else {
+		tail := &decoratedSerializeHandler{nil, m}
+		s.tail.Next = tail
+		s.tail = tail
+	}
+
+	return nil
 }
 
 // Insert injects the middleware relative to an existing middleware ID.
 // Returns error if the original middleware does not exist, or the middleware
 // being added already exists.
 func (s *SerializeStep) Insert(m SerializeMiddleware, relativeTo string, pos RelativePosition) error {
-	return s.ids.Insert(m, relativeTo, pos)
+	var prev, found *decoratedSerializeHandler
+	for h := s.head; h != nil; {
+		if h.With.ID() == relativeTo {
+			found = h
+			break
+		}
+		prev = h
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedSerializeHandler)
+	}
+	if found == nil {
+		return fmt.Errorf("not found: %s", m.ID())
+	}
+
+	if pos == Before {
+		if prev == nil { // at the front
+			s.head = &decoratedSerializeHandler{s.head, m}
+		} else { // somewhere in the middle
+			prev.Next = &decoratedSerializeHandler{found, m}
+		}
+	} else {
+		if found.Next == nil { // at the end
+			tail := &decoratedSerializeHandler{nil, m}
+			s.tail.Next = tail
+			s.tail = tail
+		} else { // somewhere in the middle
+			found.Next = &decoratedSerializeHandler{found.Next, m}
+		}
+	}
+
+	return nil
 }
 
 // Swap removes the middleware by id, replacing it with the new middleware.
 // Returns the middleware removed, or error if the middleware to be removed
 // doesn't exist.
 func (s *SerializeStep) Swap(id string, m SerializeMiddleware) (SerializeMiddleware, error) {
-	removed, err := s.ids.Swap(id, m)
-	if err != nil {
-		return nil, err
+	for h := s.head; h != nil; {
+		if h.With.ID() == id {
+			swapped := h.With
+			h.With = m
+			return swapped, nil
+		}
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedSerializeHandler)
 	}
-
-	return removed.(SerializeMiddleware), nil
+	return nil, fmt.Errorf("not found: %s", m.ID())
 }
 
 // Remove removes the middleware by id. Returns error if the middleware
 // doesn't exist.
 func (s *SerializeStep) Remove(id string) (SerializeMiddleware, error) {
-	removed, err := s.ids.Remove(id)
-	if err != nil {
-		return nil, err
+	var prev, found *decoratedSerializeHandler
+	for h := s.head; h != nil; {
+		if h.With.ID() == id {
+			found = h
+			break
+		}
+		prev = h
+		if h.Next == nil {
+			break
+		}
+		h = h.Next.(*decoratedSerializeHandler)
+	}
+	if found == nil {
+		return nil, fmt.Errorf("not found: %s", id)
+	}
+
+	if s.head == s.tail { // it's the only one
+		s.head = nil
+		s.tail = nil
+	} else if found == s.head { // at the front
+		s.head = s.head.Next.(*decoratedSerializeHandler)
+	} else if found == s.tail { // at the end
+		prev.Next = nil
+		s.tail = prev
+	} else {
+		prev.Next = found.Next // somewhere in the middle
 	}
 
-	return removed.(SerializeMiddleware), nil
+	return found.With, nil
 }
 
 // List returns a list of the middleware in the step.
 func (s *SerializeStep) List() []string {
-	return s.ids.List()
+	var ids []string
+	for h := s.head; h != nil; {
+		ids = append(ids, h.With.ID())
+		if h.Next == nil {
+			break
+		}
+
+		// once executed, tail.Next of the list will be set to an
+		// *serializeWrapHandler, make sure to check for that
+		if hnext, ok := h.Next.(*decoratedSerializeHandler); ok {
+			h = hnext
+		}
+	}
+	return ids
 }
 
 // Clear removes all middleware in the step.
 func (s *SerializeStep) Clear() {
-	s.ids.Clear()
+	s.head = nil
+	s.tail = nil
 }
 
 type serializeWrapHandler struct {
@@ -184,7 +280,7 @@ type serializeWrapHandler struct {
 
 var _ SerializeHandler = (*serializeWrapHandler)(nil)
 
-// Implements SerializeHandler, converts types and delegates to underlying
+// HandleSerialize implements SerializeHandler, converts types and delegates to underlying
 // generic handler.
 func (w serializeWrapHandler) HandleSerialize(ctx context.Context, in SerializeInput) (
 	out SerializeOutput, metadata Metadata, err error,
@@ -208,12 +304,12 @@ func (h decoratedSerializeHandler) HandleSerialize(ctx context.Context, in Seria
 	return h.With.HandleSerialize(ctx, in, h.Next)
 }
 
-// SerializeHandlerFunc provides a wrapper around a function to be used as a serialize middleware handler.
+// SerializeHandlerFunc provides a wrapper around a function to be used as serializeMiddleware.
 type SerializeHandlerFunc func(context.Context, SerializeInput) (SerializeOutput, Metadata, error)
 
 // HandleSerialize calls the wrapped function with the provided arguments.
-func (s SerializeHandlerFunc) HandleSerialize(ctx context.Context, in SerializeInput) (SerializeOutput, Metadata, error) {
-	return s(ctx, in)
+func (f SerializeHandlerFunc) HandleSerialize(ctx context.Context, in SerializeInput) (SerializeOutput, Metadata, error) {
+	return f(ctx, in)
 }
 
 var _ SerializeHandler = SerializeHandlerFunc(nil)