Indicateurs clés de performance Web
/SKILLOptimisez les indicateurs Core Web Vitals (LCP, INP, CLS) pour améliorer l'expérience utilisateur et le référencement naturel.
name: core-web-vitals
description: Optimize Core Web Vitals (LCP, INP, CLS) for better page experience and search ranking. Use when asked to "improve Core Web Vitals", "fix LCP", "reduce CLS", "optimize INP", "page experience optimization", or "fix layout shifts".
license: MIT
metadata:
author: web-quality-skills
version: "1.0"
Core Web Vitals optimization
Targeted optimization for the three Core Web Vitals metrics that affect Google Search ranking and user experience.
The three metrics
| Metric | Measures | Good | Needs work | Poor |
|---|---|---|---|---|
| LCP | Loading | ≤ 2.5s | 2.5s – 4s | > 4s |
| INP | Interactivity | ≤ 200ms | 200ms – 500ms | > 500ms |
| CLS | Visual Stability | ≤ 0.1 | 0.1 – 0.25 | > 0.25 |
Google measures at the 75th percentile — 75% of page visits must meet "Good" thresholds.
LCP: Largest Contentful Paint
LCP measures when the largest visible content element renders. Usually this is:
- Hero image or video
- Large text block
- Background image
<svg>element
Common LCP issues
1. Slow server response (TTFB > 800ms)
Fix: CDN, caching, optimized backend, edge rendering2. Render-blocking resources
<!-- ❌ Blocks rendering -->
<link rel="stylesheet" href="/all-styles.css">
<!-- ✅ Critical CSS inlined, rest deferred -->
<style>/* Critical above-fold CSS */</style>
<link rel="preload" href="/styles.css" as="style"
onload="this.onload=null;this.rel='stylesheet'">3. Slow resource load times
<!-- ❌ No hints, discovered late -->
<img src="/hero.jpg" alt="Hero">
<!-- ✅ Preloaded with high priority -->
<link rel="preload" href="/hero.webp" as="image" fetchpriority="high">
<img src="/hero.webp" alt="Hero" fetchpriority="high">4. Client-side rendering delays
// ❌ Content loads after JavaScript
useEffect(() => {
fetch('/api/hero-text').then(r => r.json()).then(setHeroText);
}, []);
// ✅ Server-side or static rendering
// Use SSR, SSG, or streaming to send HTML with content
export async function getServerSideProps() {
const heroText = await fetchHeroText();
return { props: { heroText } };
}5. Make navigations instant with the Speculation Rules API
For most sites, the LCP a user actually experiences is dominated by the next page they navigate to, not the one they landed on. Telling the browser to prerender likely-next pages on hover collapses that LCP to ~0ms.
<script type="speculationrules">
{
"prerender": [{
"where": { "href_matches": "/*" },
"eagerness": "moderate"
}]
}
</script>eagerness settings (cheapest → most aggressive): conservative (start on pointerdown), moderate (start after ~200ms hover), eager (start as soon as the link is in the viewport), immediate (start on page load). Start with moderate — it captures most navigations without prerendering pages users never visit.
Caveats:
- Bandwidth/CPU cost. Each prerender is roughly a full page load. Scope
wherecarefully (href_matchespatterns, exclude logout/checkout) and avoidimmediateoutside small sites. - Side effects fire early. Analytics, ads, and any code that runs on load will fire when the prerender starts, not when the user navigates. Gate side effects on the [
prerenderingchangeevent](https://developer.chrome.com/docs/web-platform/prerender-pages#detectwhenapageisprerenderedorusedforafull_navigation) ordocument.prerendering. - Chromium-only. Safari and Firefox ignore the script — it's a progressive enhancement, never a regression.
LCP optimization checklist
- [ ] TTFB < 800ms (use CDN, edge caching)
- [ ] LCP image preloaded with fetchpriority="high"
- [ ] LCP image optimized (WebP/AVIF, correct size)
- [ ] Critical CSS inlined (< 14KB)
- [ ] No render-blocking JavaScript in <head>
- [ ] Fonts don't block text rendering (font-display: swap)
- [ ] LCP element in initial HTML (not JS-rendered)
- [ ] Speculation Rules added for likely-next navigations (moderate eagerness)LCP element identification
// Find your LCP element
new PerformanceObserver((list) => {
const entries = list.getEntries();
const lastEntry = entries[entries.length - 1];
console.log('LCP element:', lastEntry.element);
console.log('LCP time:', lastEntry.startTime);
}).observe({ type: 'largest-contentful-paint', buffered: true });INP: Interaction to Next Paint
INP measures responsiveness across ALL interactions (clicks, taps, key presses) during a page visit. It reports the worst interaction (at 98th percentile for high-traffic pages).
INP breakdown
Total INP = Input Delay + Processing Time + Presentation Delay
| Phase | Target | Optimization |
|---|---|---|
| Input Delay | < 50ms | Reduce main thread blocking |
| Processing | < 100ms | Optimize event handlers |
| Presentation | < 50ms | Minimize rendering work |
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