Core Web Vitals are measurements that describe important parts of the page experience. They focus on whether content appears quickly, stays stable, and responds when visitors interact.
Largest Contentful Paint
Largest Contentful Paint, or LCP, measures when the largest important visible element appears. On many sites this is a hero image, headline block, or large section near the top of the page.
Slow LCP often points to large images, render-blocking CSS, slow hosting, or complicated above-the-fold layouts.
Cumulative Layout Shift
Cumulative Layout Shift, or CLS, measures unexpected movement. If the page jumps while a visitor is reading or trying to click, the experience feels unreliable.
Images without dimensions, ads, embeds, banners, and late-loading fonts are common causes.
Responsiveness
Responsiveness metrics help show whether the page is busy running scripts when the visitor wants to interact. Heavy JavaScript can make a page look loaded while still feeling delayed.
Lab and field data
A speed test often shows lab data from a controlled test. Field data comes from real visitors when enough data is available. Lab data helps diagnose problems; field data helps confirm what users experience.
Practical first steps
- Optimize the largest above-the-fold image.
- Reserve space for images and embeds.
- Reduce unnecessary JavaScript.
- Improve caching and server response time.
- Retest important pages after each change.