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. 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.

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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">