core-web-vitals

Files (4)

• SKILL.md
• CLS.md · references/
• INP.md · references/
• LCP.md · references/

SKILL.md (first lines)

# Core Web Vitals optimization

Targeted optimization for the three Core Web Vitals using field data to identify user impact and browser traces to diagnose causes.

## Measure before optimizing

When a runnable URL is available, read [the performance measurement workflow](../performance/references/MEASUREMENT.md). Prefer this sequence:

1. Check page-level CrUX p75 data, with a clearly labeled origin fallback when page data is unavailable.
2. Record a browser performance trace under stated conditions. With Chrome DevTools MCP, trace summaries can include CrUX alongside the observed lab metrics.
3. Analyze only the insights associated with the failing metric, then inspect the implicated code and resources.
4. Re-run equivalent lab measurements after the fix. Do not claim an immediate field improvement; CrUX and first-party RUM need new user visits.

If only source code is available, identify likely causes but do not claim that LCP, INP, or CLS is failing without runtime evidence.

## 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 rendering
```

**2. Render-blocking resources**
```html
<!-- ❌ 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**
```html
<!-- ❌ LCP image is discovered only after a stylesheet loads -->
<div class="hero"></div>

<!-- ✅ Discoverable in initial HTML and prioritized -->
<link rel="preload" href="/hero.webp" as="image" fetchpriority="high">
<img src="/hero.webp" alt="Hero" fetchpriority="high">
```

Prefer a discoverable `<img>` with `fetchpriority="high"`. Add the preload only when the trace shows that the resource would otherwise be discovered late; duplicate or speculative preloads can compete for bandwidth.

**4. Client-side rendering delays**
```javascript
// ❌ 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 sites with predictable same-origin journeys, prerendering a likely next page can make a successful subsequent navigation much faster. Treat this as a measured navigation optimization, not a substitute for fixing the current page's LCP.

```html
<script type="speculationrules">
{
  "prerender": [{
    "where": { "href_matches": "/*" },
    "eagerness": "moderate"
  }]
}
</script>
```

Current Chrome behavior is specific enough to guide the choice:

| `eagerness` | Trigger |
|-------------|---------|
| `conservative` | Pointer or touch down |
| `moderate` | Desktop: 200ms hover, or earlier pointer down; mobile: viewport heuristics |
| `eager` | Chrome 143+: desktop 10ms hover; mobile 50ms after the anchor enters the viewport |
| `immediate` | As soon as the rules are observed |

Start conservatively and measure prediction hit rate, transferred bytes, server load, and navigation improvement before expanding the rules. Recheck [Chrome's maintained eagerness documentation](https://developer.chrome.com/docs/web-platform/prerender-pages#eagerness) before hardcoding timing-sensitive behavior.

Caveats:
- **Bandwidth/CPU cost.** Each prerender is roughly a full page load. Scope `where` carefully (`href_matches` patterns, exclude logout/checkout) and avoid `immediate` outside 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 [`prerenderingchange` event](https://developer.chrome.com/docs/web-platform/prerender-pages#detect_when_a_page_is_prerendered_or_used_for_a_full_navigation) or `document.prerendering`.
- **Chromium-only.** Safari and Firefox ignore the script — it's a progressive enhancement, never a regression.

### LCP optimization checklist

```markdown
- [ ] TTFB < 800ms (use CDN, edge caching)
- [ ] LCP resource is discoverable in initial HTML and prioritized; preload only if the trace shows late discovery
- [ ] LCP image optimized (WebP/AVIF, correct size)
- [ ] Critical CSS inlined (< 14KB)
- [ ] No render-blocking JavaScript in <head>

From duart-555/claude-code-setup, MIT.

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