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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

Step 0 — is the kit here? This skill reads files from the kit. Check once: ls ${CLAUDE_SKILL_DIR}/../../../tokens >/dev/null 2>&1 && echo KIT_OK || echo KIT_MISSING On KIT_MISSING only the skill folders were installed, which is what npx skills add does. Say so plainly, point the user at npx ux-ui-agent-skills init or the plugin install, and stop. Do not guess the contents of a file you could not open.

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

Steps

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