perf: lazy eval headers from the requestStore

packages/next/client/components/hooks-server.ts

@@ -24,7 +24,7 @@ export function headers(): Headers {
       ? requestAsyncStorage.getStore()!
       : requestAsyncStorage
 
-  return requestStore.headers
+  return requestStore.headers()
 }
 
 export function previewData() {

packages/next/client/components/request-async-storage.ts

@@ -2,7 +2,7 @@ import type { AsyncLocalStorage } from 'async_hooks'
 import { Cookies } from '../../server/web/spec-extension/cookies'
 
 export interface RequestStore {
-  headers: Headers
+  headers: () => Headers
   cookies: Cookies
   previewData: any
 }

packages/next/server/app-render.tsx

@@ -83,6 +83,17 @@ const INTERNAL_COOKIES_INSTANCE = Symbol('internal for cookies readonly')
 function readonlyCookiesError() {
   return new Error('ReadonlyCookies cannot be modified')
 }
+
+function lazy<T>(fn: () => T) {
+  let value: any
+  return () => {
+    if (!value) {
+      value = fn()
+    }
+    return value
+  }
+}
+
 class ReadonlyNextCookies {
   [INTERNAL_COOKIES_INSTANCE]: NextCookies
 
@@ -1572,7 +1583,7 @@ export async function renderToHTMLOrFlight(
   )
 
   const requestStore = {
-    headers: new ReadonlyHeaders(headersWithoutFlight(req.headers)),
+    headers: lazy(() => new ReadonlyHeaders(headersWithoutFlight(req.headers))),
     cookies: new ReadonlyNextCookies({
       headers: {
         get: (key) => {