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+# Performance Checklist
+
+Quick reference checklist for web application performance. Use alongside the `performance-optimization` skill.
+
+## Table of Contents
+
+- [Core Web Vitals Targets](#core-web-vitals-targets)
+- [TTFB Diagnosis](#ttfb-diagnosis)
+- [Frontend Checklist](#frontend-checklist)
+- [Backend Checklist](#backend-checklist)
+- [Measurement Commands](#measurement-commands)
+- [Common Anti-Patterns](#common-anti-patterns)
+
+## Core Web Vitals Targets
+
+| Metric | Good | Needs Work | Poor |
+|--------|------|------------|------|
+| LCP (Largest Contentful Paint) | ≤ 2.5s | ≤ 4.0s | > 4.0s |
+| INP (Interaction to Next Paint) | ≤ 200ms | ≤ 500ms | > 500ms |
+| CLS (Cumulative Layout Shift) | ≤ 0.1 | ≤ 0.25 | > 0.25 |
+
+## TTFB Diagnosis
+
+When TTFB is slow (> 800ms), check each component in DevTools Network waterfall:
+
+- [ ] **DNS resolution** slow → add `<link rel="dns-prefetch">` or `<link rel="preconnect">` for known origins
+- [ ] **TCP/TLS handshake** slow → enable HTTP/2, consider edge deployment, verify keep-alive
+- [ ] **Server processing** slow → profile backend, check slow queries, add caching
+
+## Frontend Checklist
+
+### Images
+- [ ] Images use modern formats (WebP, AVIF)
+- [ ] Images are responsively sized (`srcset` and `sizes`)
+- [ ] Images and `<source>` elements have explicit `width` and `height` (prevents CLS in art direction)
+- [ ] Below-the-fold images use `loading="lazy"` and `decoding="async"`
+- [ ] Hero/LCP images use `fetchpriority="high"` and no lazy loading
+
+### JavaScript
+- [ ] Bundle size under 200KB gzipped (initial load)
+- [ ] Code splitting with dynamic `import()` for routes and heavy features
+- [ ] Tree shaking enabled (verify dependency ships ESM and marks `sideEffects: false`)
+- [ ] No blocking JavaScript in `<head>` (use `defer` or `async`)
+- [ ] Heavy computation offloaded to Web Workers (if applicable)
+- [ ] `React.memo()` on expensive components that re-render with same props
+- [ ] `useMemo()` / `useCallback()` only where profiling shows benefit
+- [ ] Long tasks (> 50ms) broken up to keep the main thread available — main lever for INP
+- [ ] `yieldToMain` pattern used inside long-running loops so input events can run between chunks
+- [ ] Modern scheduling APIs used where available: `scheduler.yield()` (preferred), `scheduler.postTask()` with priorities, `isInputPending()` to yield only when needed
+- [ ] `requestIdleCallback` for deferrable, non-urgent work (analytics flush, prefetch, warmup)
+- [ ] Non-critical work deferred out of event handlers (e.g. analytics, logging) so the response to the interaction is not delayed
+- [ ] Third-party scripts loaded with `async` / `defer`, audited for size, and fronted by a facade when heavy (chat widgets, embeds)
+
+### CSS
+- [ ] Critical CSS inlined or preloaded
+- [ ] No render-blocking CSS for non-critical styles
+- [ ] No CSS-in-JS runtime cost in production (use extraction)
+
+### Fonts
+- [ ] Limited to 2–3 font families, 2–3 weights each (every additional weight is another request)
+- [ ] WOFF2 format only (smallest, universal support — skip WOFF/TTF/EOT)
+- [ ] Self-hosted when possible (third-party font CDNs add DNS + TCP + TLS round-trips)
+- [ ] LCP-critical fonts preloaded: `<link rel="preload" as="font" type="font/woff2" crossorigin>`
+- [ ] `font-display: swap` (or `optional` for non-critical) to avoid FOIT blocking render
+- [ ] Subsetted via `unicode-range` to ship only the glyphs each page needs
+- [ ] Variable fonts considered when multiple weights/styles are required (one file replaces many)
+- [ ] Fallback font metrics adjusted with `size-adjust`, `ascent-override`, `descent-override` to reduce CLS on font swap
+- [ ] System font stack considered before any custom font
+
+### Network
+- [ ] Static assets cached with long `max-age` + content hashing
+- [ ] API responses cached where appropriate (`Cache-Control`)
+- [ ] HTTP/2 or HTTP/3 enabled
+- [ ] Resources preconnected (`<link rel="preconnect">`) for known origins
+- [ ] `fetchpriority` used on critical non-image resources (e.g., key `<link rel="preload">`, above-the-fold `<script>`) — not only on `<img>`
+- [ ] No unnecessary redirects
+
+### Rendering
+- [ ] No layout thrashing (forced synchronous layouts)
+- [ ] Animations use `transform` and `opacity` (GPU-accelerated)
+- [ ] Long lists use virtualization (e.g., `react-window`)
+- [ ] No unnecessary full-page re-renders
+- [ ] Off-screen sections use `content-visibility: auto` with `contain-intrinsic-size` to skip layout/paint of non-visible areas
+- [ ] No `unload` event handlers and no `Cache-Control: no-store` on HTML responses — preserves back/forward cache (bfcache) eligibility
+
+## Backend Checklist
+
+### Database
+- [ ] No N+1 query patterns (use eager loading / joins)
+- [ ] Queries have appropriate indexes
+- [ ] List endpoints paginated (never `SELECT * FROM table`)
+- [ ] Connection pooling configured
+- [ ] Slow query logging enabled
+
+### API
+- [ ] Response times < 200ms (p95)
+- [ ] No synchronous heavy computation in request handlers
+- [ ] Bulk operations instead of loops of individual calls
+- [ ] Response compression (gzip/brotli)
+- [ ] Appropriate caching (in-memory, Redis, CDN)
+
+### Infrastructure
+- [ ] CDN for static assets
+- [ ] Server located close to users (or edge deployment)
+- [ ] Horizontal scaling configured (if needed)
+- [ ] Health check endpoint for load balancer
+
+## Measurement Commands
+
+### INP field data and DevTools workflow
+
+1. **Field data first** — check [CrUX Vis](https://developer.chrome.com/docs/crux/vis) or your RUM tool for real-user INP before optimising
+2. **Identify slow interactions** — open DevTools → Performance panel → record while interacting; look for long tasks triggered by clicks/keystrokes
+3. **Test on mid-range Android** — INP issues often only surface on slower hardware; use a real device or DevTools CPU throttling (4×–6× slowdown)
+
+```bash
+# Lighthouse CLI
+npx lighthouse https://localhost:3000 --output json --output-path ./report.json
+
+# Bundle analysis
+npx webpack-bundle-analyzer stats.json
+# or for Vite:
+npx vite-bundle-visualizer
+
+# Check bundle size
+npx bundlesize
+
+# Web Vitals in code
+import { onLCP, onINP, onCLS } from 'web-vitals';
+onLCP(console.log);
+onINP(console.log);
+onCLS(console.log);
+
+# INP with interaction-level detail (attribution build)
+import { onINP } from 'web-vitals/attribution';
+onINP(({ value, attribution }) => {
+  const { interactionTarget, inputDelay, processingDuration, presentationDelay } = attribution;
+  console.log({ value, interactionTarget, inputDelay, processingDuration, presentationDelay });
+});
+```
+
+## Common Anti-Patterns
+
+| Anti-Pattern | Impact | Fix |
+|---|---|---|
+| N+1 queries | Linear DB load growth | Use joins, includes, or batch loading |
+| Unbounded queries | Memory exhaustion, timeouts | Always paginate, add LIMIT |
+| Missing indexes | Slow reads as data grows | Add indexes for filtered/sorted columns |
+| Layout thrashing | Jank, dropped frames | Batch DOM reads, then batch writes |
+| Unoptimized images | Slow LCP, wasted bandwidth | Use WebP, responsive sizes, lazy load |
+| Large bundles | Slow Time to Interactive | Code split, tree shake, audit deps |
+| Blocking main thread | Poor INP, unresponsive UI | Chunk long tasks with `scheduler.yield()` / `yieldToMain`, offload to Web Workers |
+| Memory leaks | Growing memory, eventual crash | Clean up listeners, intervals, refs |