perf: skip reactive overhead during SSR and avoid V8 hidden class mutations#91562
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perf: skip reactive overhead during SSR and avoid V8 hidden class mutations#91562benfavre wants to merge 2 commits intovercel:canaryfrom
benfavre wants to merge 2 commits intovercel:canaryfrom
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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>
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The delete→undefined changes for NEXT_URL header and request-meta are already in PR vercel#91560. Keep only the changes unique to this PR: - parsedUrl.query fresh object replacement (avoids N delete operations) - loadable timer guards during SSR
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Summary
Two targeted optimizations for SSR hot paths, inspired by TanStack Start's 5.5x SSR throughput improvements.
1. Replace loop-delete with fresh object for query clearing
In
base-server.ts, the invoke path handler clearsparsedUrl.queryby loop-deleting every key, then repopulating viaObject.assign. This performs Ndeleteoperations, each mutating the V8 hidden class. Replaced with a single object assignment:```ts
// Before: N delete operations + Object.assign
for (const key of Object.keys(parsedUrl.query)) { delete parsedUrl.query[key] }
if (invokeQuery) { Object.assign(parsedUrl.query, invokeQuery) }
// After: single assignment
parsedUrl.query = invokeQuery ? { ...invokeQuery } : {}
```
2. Skip loadable timer setup during SSR
`LoadableSubscription.retry()` creates `setTimeout` timers for delay/timeout states. During SSR, `useSyncExternalStore` uses the server snapshot directly without subscribing, so these timers fire with no listeners and leak in the Node.js process. Added `typeof window !== 'undefined'` guards.
Test plan
🤖 Generated with Claude Code