web-performance-checklistlisted

# Web Performance Checklist A concrete checklist for diagnosing and improving web performance. Measure before and after — optimize against field data, not hunches. ## Rationalizations (read first) Pre-written rebuttals to the excuses that precede a skipped measurement. If you catch yourself thinking the left column, the right column is the answer. | Rationalization | Rebuttal | |---|---| | "It's fast on my machine." | Your machine isn't the p75 user. Check field data (CrUX/RUM) and throttle to mid-tier Android. | | "I'll add the index later." | An unindexed query degrades with data growth — later is an incident, not a task. | | "Memoize everything to be safe." | `useMemo`/`useCallback` cost too. Without a profile they add overhead, not speed. | | "The bundle's only a bit bigger." | Bloat compounds. Read the analyzer, not your gut — name the KB before adding the dep. | | "It looks instant locally." | Local has no network or CPU throttle. Unverified under realistic conditions = unverified. | ## Core Web Vitals targets | Metric | Good | Needs improvement | 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 too high (> 800ms), check each component in the Network panel: - [ ] Slow DNS → add `<link rel="dns-prefetch">` or `preconnect` for known origins. - [ ] Slow TCP/TLS handshake →