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Core Web Vitals: What Engineers Need to Know

Core Web Vitals are the only page-experience metrics Google has confirmed as ranking signals. Here is what they measure and how to fix them.

The three metrics

Largest Contentful Paint (LCP)

How long until the largest visible element finishes rendering. Target: under 2.5 seconds. The largest element is usually a hero image, a heading, or a large text block.

Common causes of bad LCP:

  • Slow server response (TTFB > 800ms)
  • Render-blocking CSS or JavaScript
  • Unoptimized images (no width/height, no modern formats)
  • Web fonts that block rendering

Cumulative Layout Shift (CLS)

How much the page layout jumps around as it loads. Target: under 0.1. Every time an element moves after the user can see the page, that is layout shift.

Common causes of bad CLS:

  • Images without dimensions (the browser does not know how much space to reserve)
  • Ads or embeds that load late and push content down
  • Web fonts that swap and change text size
  • Dynamically injected content above the fold

Interaction to Next Paint (INP)

How long the page takes to respond to user input. Target: under 200ms. This replaced First Input Delay (FID) in March 2024.

Common causes of bad INP:

  • Long JavaScript tasks blocking the main thread
  • Large DOM size (10,000+ nodes)
  • Synchronous layout operations triggered by JavaScript

How to measure

Lab data (Lighthouse, WebPageTest) tells you what could happen. Field data (Chrome User Experience Report / CrUX) tells you what actually happened to real users. Rankings use field data. Google provides the CrUX dataset and PageSpeed Insights for this.

Google Search Console reports Core Web Vitals grouped by similar URLs. PageSpeed Insights shows both lab and field data for a specific URL.

The engineering approach

Do not chase Lighthouse scores. Instead:

  1. Check CrUX data for your actual URLs
  2. Identify the URLs with real-user failures
  3. Reproduce in lab tools to diagnose
  4. Fix the root cause, not the symptom
  5. Verify the fix in the next CrUX collection cycle (28 days)

Core Web Vitals are a floor, not a ceiling. Meeting the thresholds removes a penalty — it does not add a bonus.

Related: What Is Technical SEO and Why It Matters covers the full technical SEO landscape that Core Web Vitals sit within.