core-web-vitals

1# Core Web Vitals optimization
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3Targeted optimization for the three Core Web Vitals using field data to identify user impact and browser traces to diagnose causes.
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5## Measure before optimizing
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7When a runnable URL is available, read [the performance measurement workflow](../performance/references/MEASUREMENT.md). Prefer this sequence:
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91. Check page-level CrUX p75 data, with a clearly labeled origin fallback when page data is unavailable.
102. Record a browser performance trace under stated conditions. With Chrome DevTools MCP, trace summaries can include CrUX alongside the observed lab metrics.
113. Analyze only the insights associated with the failing metric, then inspect the implicated code and resources.
124. Re-run equivalent lab measurements after the fix. Do not claim an immediate field improvement; CrUX and first-party RUM need new user visits.
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14If only source code is available, identify likely causes but do not claim that LCP, INP, or CLS is failing without runtime evidence.
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16## The three metrics
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18| Metric | Measures | Good | Needs work | Poor |
19|--------|----------|------|------------|------|
20| **LCP** | Loading | ≤ 2.5s | 2.5s – 4s | > 4s |
21| **INP** | Interactivity | ≤ 200ms | 200ms – 500ms | > 500ms |
22| **CLS** | Visual Stability | ≤ 0.1 | 0.1 – 0.25 | > 0.25 |
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24Google measures at the **75th percentile** — 75% of page visits must meet "Good" thresholds.
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26---
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28## LCP: Largest Contentful Paint
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