# Performance

> 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: `majiayu000/performance-2` (Agent Skill, multi-file: 2 files)
- Install (CLI): `npx skillmds add majiayu000/performance-2`
- Raw SKILL.md: https://api.skillmd.com/api/skills/majiayu000/performance-2/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: majiayu000 (https://skillmd.com/u/majiayu000)
- Updated: 2026-09-09
- Page: https://skillmd.com/skills/majiayu000/performance-2

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# 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.