version: "1.0.1" name: performance-optimization description: "Profiles, reduces frontend/backend costs: split bundles, optimize assets, apply caching, fix Core Web Vitals regressions. Use when profiling Lighthouse/CI regressions, reducing bundle size, or fixing high CLS/LCP/TTI metrics."

Performance Optimization

Measure before changing anything. Add React.memo/useMemo/useCallback only after a profile shows the cost — never speculatively. Set budgets (load time, memory, API latency) and enforce them in CI.

Rules

• Node: async APIs only — never readFileSync or other sync I/O on a request path.
• Debounce input-driven fetches at 300 ms.
• INP replaced FID as a Core Web Vital in March 2024: budget it at ≤200 ms, and measure every interaction, not just the first.
• Profile Node with clinic.js or node --inspect; profile React with <Profiler onRender> or the DevTools Profiler.

Profiling Workflow

1. Lighthouse (or the CI perf job) for a baseline; name the failing metric (LCP/CLS/INP/TTI). Lighthouse emulates mobile by default; pass --preset=desktop when the regression is desktop-only.
2. Profile to locate the hotspot call stacks / long tasks.
3. Apply the minimal fix (code-split, memoize, shrink payloads, defer non-critical work); confirm in the profiler that the measured hotspot actually shrank.
4. Re-run Lighthouse / the CI perf job. Ship only at ≥10% improvement or once inside budget.
5. If the regression persists, iterate and record a rollback plan; note fixes in the changelog.