core-web-vitals

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.

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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 rendering

2. 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.