Verify a bottleneck exists before touching performance. Use before any caching, memoization, query rewrite, algorithm swap, or "this seems slow" change; when a spec sets a performance target or Core Web Vitals threshold; when a user reports slowness; or when a PR bundles a performance claim with no before/after number. Skip for a fix where the bottleneck is already obvious from the code itself (e.g. an accidental O(n²) on a hot path).
