performance

name	performance
description	Optimize UI performance against Core Web Vitals — LCP, INP, CLS — with loading/code-split strategy, layout-shift prevention, and animation performance rules. Use when the user wants to improve speed, fix jank or layout shift, hit Web Vitals budgets, or make a UI feel fast on low-end devices.

Skill: Performance

Make the UI fast and stable. Treat performance as an accessibility concern — slow/janky UIs fail low-end devices first.

Steps

1. Read workflows/performance.md (Core Web Vitals targets, loading strategy, layout-shift, animation perf, design-system runtime cost).
2. Diagnose against budgets: LCP ≤ 2.5s, INP ≤ 200ms, CLS ≤ 0.1. Measure on a mid-tier mobile profile (throttle slow 4G + 4× CPU).
3. Loading: render above-fold first (SSR/Server Components), code-split by route + heavy widget, lazy-load below-fold/behind-interaction (Astro islands / Qwik), preload one critical font weight, modern responsive images.
4. Kill layout shift: size skeletons to final dimensions, reserve media space via aspect-ratio (tokens/sizing.json), never inject content above existing.
5. Animation: only transform/opacity, will-change sparingly, 100–300ms (tokens/motion.json), honor prefers-reduced-motion. Prefer CSS state styling over JS for low INP; tree-shake/per-component imports.

Verification (definition of done)

• Lighthouse / field data meets LCP ≤ 2.5s, INP ≤ 200ms, CLS ≤ 0.1 on mid-tier mobile.
• First interaction stays responsive under slow-4G + 4× CPU throttle.
• Skeletons + media reserve space (zero CLS on load); no non-compositor animation in loops.