perf(server): avoid delete on hot-path objects to preserve V8 hidden classes

Using `delete` on object properties changes the V8 hidden class (object
"shape"), forcing the engine into dictionary/slow-property mode and
disabling inline caching for all subsequent property accesses on that
object. TanStack observed >50% CPU time reduction in affected methods
after switching to `undefined` assignment.

This commit makes three targeted changes in the SSR request hot path:

1. **`parsedUrl.query` clear-and-replace** (base-server.ts): Instead of
   loop-deleting every key then using `Object.assign`, reassign
   `parsedUrl.query` to a fresh object. This avoids N shape transitions
   and is semantically equivalent since `parsedUrl.query` is accessed
   via the property, not by captured reference.

2. **`req.headers[NEXT_URL]` removal** (base-server.ts): Use
   `= undefined` instead of `delete`. Downstream code accesses this
   header by value (`headers[NEXT_URL]`), never with the `in` operator,
   so `undefined` is semantically equivalent.

3. **`removeRequestMeta`** (request-meta.ts): Use `= undefined` instead
   of `delete`. All consumers use `getRequestMeta(req, key)` which
   returns `meta[key]` — `undefined` is indistinguishable from a
   missing key. The request meta object persists for the entire request
   lifetime, so preserving its shape benefits all downstream accesses.

Several other `delete` sites were analyzed and intentionally left
unchanged because they affect objects that are later serialized via
`querystring.stringify` or `url.format`, where `undefined` values
would leak as empty query parameters (e.g., `_rsc=`).

Reference: https://tanstack.com/blog/tanstack-start-v1-5x-faster-ssr

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: benfavre

packages/next/src/server/base-server.ts

@@ -1541,14 +1541,12 @@ export default abstract class Server<
         }
         parsedUrl.pathname = normalizeResult.pathname
 
-        for (const key of Object.keys(parsedUrl.query)) {
-          delete parsedUrl.query[key]
-        }
+        // Replace the query object instead of loop-deleting every key.
+        // Using `delete` on object properties mutates V8 hidden classes,
+        // which de-optimises inline caches on downstream property accesses.
+        // See: https://tanstack.com/blog/tanstack-start-v1-5x-faster-ssr
         const invokeQuery = getRequestMeta(req, 'invokeQuery')
-
-        if (invokeQuery) {
-          Object.assign(parsedUrl.query, invokeQuery)
-        }
+        parsedUrl.query = invokeQuery ? { ...invokeQuery } : {}
 
         finished = await this.normalizeAndAttachMetadata(req, res, parsedUrl)
         if (finished) return
@@ -2018,7 +2016,9 @@ export default abstract class Server<
     if (!addedNextUrlToVary) {
       // Remove `Next-URL` from the request headers we determined it wasn't necessary to include in the Vary header.
       // This is to avoid any dependency on the `Next-URL` header being present when preparing the response.
-      delete req.headers[NEXT_URL]
+      // Use undefined assignment instead of delete to preserve V8 hidden class.
+      // Downstream code checks this header by value, not with the `in` operator.
+      req.headers[NEXT_URL] = undefined
     }
   }
 

packages/next/src/server/request-meta.ts

@@ -372,7 +372,10 @@ export function removeRequestMeta<K extends keyof RequestMeta>(
   key: K
 ) {
   const meta = getRequestMeta(request)
-  delete meta[key]
+  // Set to undefined instead of delete to preserve V8 hidden class.
+  // All consumers use getRequestMeta(req, key) which returns meta[key],
+  // so undefined is semantically equivalent to a missing key.
+  meta[key] = undefined as RequestMeta[K]
   return setRequestMeta(request, meta)
 }