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Core Web Vitals – Optimizing for Google’s UX Metrics

July 10, 2021

Core Web Vitals are a set of metrics defined by Google to evaluate the user experience of a web page. They focus on loading speed, interactivity, and visual stability.

These three metrics became especially important for front-end developers when they started to influence search ranking.

What are Core Web Vitals?

  • Largest Contentful Paint (LCP): Measures loading performance. A good score is under 2.5 seconds.
  • First Input Delay (FID): Measures interactivity. A good score is under 100ms.
  • Cumulative Layout Shift (CLS): Measures visual stability. A good score is below 0.1.

Tips to Improve Core Web Vitals

Improving LCP

  • Use lazy loading for images and videos
  • Optimize critical rendering path
  • Serve images in modern formats like WebP
  • Reduce server response times

Improving FID

  • Minimize main-thread blocking time
  • Reduce JavaScript bundle size
  • Use web workers for heavy computation
  • Split code using dynamic import()

Improving CLS

  • Always include width and height on images and videos
  • Reserve space for ads and embeds
  • Avoid injecting content above existing content
  • Use CSS aspect-ratio to maintain size stability

How to Measure Core Web Vitals

You can test your site using:

  • Lighthouse (in Chrome DevTools)
  • PageSpeed Insights
  • Web Vitals Chrome Extension
  • Real-user monitoring tools
# Run Lighthouse in CLI
npm install -g lighthouse
lighthouse https://your-site.com

Improving these metrics not only helps SEO but also provides a better experience for users across devices and connection speeds.

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