perf: pre-compute Vary header and reduce per-request header overhead

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

@@ -158,6 +158,12 @@ import {
   readBodyWithSizeLimit,
 } from './lib/postponed-request-body'
 
+// Pre-compute Vary header strings at module level to avoid repeated string
+// concatenation on every request. These are constant values derived from
+// header name constants and never change at runtime.
+const STATIC_VARY_HEADER = `${RSC_HEADER}, ${NEXT_ROUTER_STATE_TREE_HEADER}, ${NEXT_ROUTER_PREFETCH_HEADER}, ${NEXT_ROUTER_SEGMENT_PREFETCH_HEADER}`
+const STATIC_VARY_HEADER_WITH_NEXT_URL = `${STATIC_VARY_HEADER}, ${NEXT_URL}`
+
 export type FindComponentsResult<
   NextModule extends GenericComponentMod = GenericComponentMod,
 > = {
@@ -1995,20 +2001,19 @@ export default abstract class Server<
     isAppPath: boolean,
     resolvedPathname: string
   ): void {
-    const baseVaryHeader = `${RSC_HEADER}, ${NEXT_ROUTER_STATE_TREE_HEADER}, ${NEXT_ROUTER_PREFETCH_HEADER}, ${NEXT_ROUTER_SEGMENT_PREFETCH_HEADER}`
     const isRSCRequest = getRequestMeta(req, 'isRSCRequest') ?? false
 
     let addedNextUrlToVary = false
 
     if (isAppPath && this.pathCouldBeIntercepted(resolvedPathname)) {
       // Interception route responses can vary based on the `Next-URL` header.
       // We use the Vary header to signal this behavior to the client to properly cache the response.
-      res.appendHeader('vary', `${baseVaryHeader}, ${NEXT_URL}`)
+      res.appendHeader('vary', STATIC_VARY_HEADER_WITH_NEXT_URL)
       addedNextUrlToVary = true
     } else if (isAppPath || isRSCRequest) {
       // We don't need to include `Next-URL` in the Vary header for non-interception routes since it won't affect the response.
       // We also set this header for pages to avoid caching issues when navigating between pages and app.
-      res.appendHeader('vary', baseVaryHeader)
+      res.appendHeader('vary', STATIC_VARY_HEADER)
     }
 
     if (!addedNextUrlToVary) {

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

@@ -378,9 +378,14 @@ export function addRequestMeta<K extends keyof RequestMeta>(
   key: K,
   value: RequestMeta[K]
 ) {
-  const meta = getRequestMeta(request)
-  meta[key] = value
-  return setRequestMeta(request, meta)
+  let meta = request[NEXT_REQUEST_META]
+  if (meta) {
+    meta[key] = value
+  } else {
+    meta = { [key]: value } as RequestMeta
+    request[NEXT_REQUEST_META] = meta
+  }
+  return meta
 }
 
 /**

packages/next/src/server/route-modules/app-page/module.ts

@@ -34,6 +34,11 @@ import type { UrlWithParsedQuery } from 'url'
 import type { IncomingMessage } from 'http'
 import { normalizeAppPageRequestUrl } from './normalize-request-url'
 
+// Pre-compute Vary header strings at module level to avoid repeated string
+// concatenation on every request.
+const STATIC_VARY_HEADER = `${RSC_HEADER}, ${NEXT_ROUTER_STATE_TREE_HEADER}, ${NEXT_ROUTER_PREFETCH_HEADER}, ${NEXT_ROUTER_SEGMENT_PREFETCH_HEADER}`
+const STATIC_VARY_HEADER_WITH_NEXT_URL = `${STATIC_VARY_HEADER}, ${NEXT_URL}`
+
 let vendoredReactRSC
 let vendoredReactSSR
 
@@ -180,18 +185,16 @@ export class AppPageRouteModule extends RouteModule<
     resolvedPathname: string,
     interceptionRoutePatterns: RegExp[]
   ): string {
-    const baseVaryHeader = `${RSC_HEADER}, ${NEXT_ROUTER_STATE_TREE_HEADER}, ${NEXT_ROUTER_PREFETCH_HEADER}, ${NEXT_ROUTER_SEGMENT_PREFETCH_HEADER}`
-
     if (
       this.pathCouldBeIntercepted(resolvedPathname, interceptionRoutePatterns)
     ) {
       // Interception route responses can vary based on the `Next-URL` header.
       // We use the Vary header to signal this behavior to the client to properly cache the response.
-      return `${baseVaryHeader}, ${NEXT_URL}`
+      return STATIC_VARY_HEADER_WITH_NEXT_URL
     } else {
       // We don't need to include `Next-URL` in the Vary header for non-interception routes since it won't affect the response.
       // We also set this header for pages to avoid caching issues when navigating between pages and app.
-      return baseVaryHeader
+      return STATIC_VARY_HEADER
     }
   }
 }

packages/next/src/server/send-payload.ts

@@ -32,6 +32,11 @@ export function sendEtagResponse(
   return false
 }
 
+// Pre-computed constant header name/value pairs to avoid repeated string
+// allocation on every response.
+const X_POWERED_BY_HEADER = 'X-Powered-By' as const
+const X_POWERED_BY_VALUE = 'Next.js' as const
+
 export async function sendRenderResult({
   req,
   res,
@@ -52,7 +57,7 @@ export async function sendRenderResult({
   }
 
   if (poweredByHeader && result.contentType === HTML_CONTENT_TYPE_HEADER) {
-    res.setHeader('X-Powered-By', 'Next.js')
+    res.setHeader(X_POWERED_BY_HEADER, X_POWERED_BY_VALUE)
   }
 
   // If cache control is already set on the response we don't