perf: skip unnecessary reactive setup during SSR

Three targeted optimizations to reduce per-request overhead during
server-side rendering:

1. loadable.shared-runtime.tsx: Skip setTimeout timer setup during SSR.
   The LoadableSubscription creates delay/timeout timers that only serve
   to update UI state for loading indicators. During SSR, React renders
   once synchronously — there are no subscribers to notify and the timers
   would leak in the Node.js process. The synchronous `delay === 0` path
   is preserved since it has no side effects.

2. request-meta.ts: Use undefined assignment instead of delete when
   removing request metadata keys. The `delete` operator causes V8 to
   create a new hidden class (shape transition), which de-optimises
   inline caches on downstream property accesses. Setting to undefined
   is semantically equivalent since consumers check via meta[key].

3. base-server.ts: Replace a delete-loop over query keys with fresh
   object creation, and use undefined assignment instead of delete for
   the Next-URL header. Both avoid V8 hidden class mutations.

Inspired by: https://tanstack.com/blog/tanstack-start-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 +1541,11 @@ 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.
         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 +2015,9 @@ 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)
 }
 

packages/next/src/shared/lib/loadable.shared-runtime.tsx

@@ -196,7 +196,10 @@ class LoadableSubscription {
       if (typeof opts.delay === 'number') {
         if (opts.delay === 0) {
           this._state.pastDelay = true
-        } else {
+        } else if (typeof window !== 'undefined') {
+          // Skip timers during SSR — the render is synchronous,
+          // there are no subscribers to notify, and the timers
+          // would leak in the Node.js process.
           this._delay = setTimeout(() => {
             this._update({
               pastDelay: true,
@@ -205,7 +208,7 @@ class LoadableSubscription {
         }
       }
 
-      if (typeof opts.timeout === 'number') {
+      if (typeof opts.timeout === 'number' && typeof window !== 'undefined') {
         this._timeout = setTimeout(() => {
           this._update({ timedOut: true })
         }, opts.timeout)