Core Web Vitals are a set of measurements, defined by Google, that describe how a page feels to a real visitor: how quickly the main content appears, how promptly the page reacts when someone interacts with it, and whether the layout stays still while loading. They belong to a wider group of page experience signals and are measured from actual user visits wherever enough data exists.
The Three Metrics
- Largest Contentful Paint (LCP) records how long the largest visible element, often a hero image or main heading block, takes to render. Google's published "good" threshold is 2.5 seconds or less
- Interaction to Next Paint (INP) measures how quickly the page responds visually after taps, clicks and key presses throughout a visit. A good score is 200 milliseconds or less. It replaced First Input Delay in 2024
- Cumulative Layout Shift (CLS) adds up unexpected movement of content as the page loads, such as a button jumping just as someone tries to press it. A good score is 0.1 or lower
Google assesses these at the 75th percentile of visits, so most visitors, not only the fastest, need a good experience for a page to pass.
Field Data and Lab Data
Field data comes from real users, collected in the Chrome User Experience Report and shown in Search Console and PageSpeed Insights. Lab data comes from a simulated test run under fixed conditions, for example in Lighthouse. Lab tests are ideal for diagnosing a problem and checking a fix because results are repeatable, but field data reflects what visitors actually experienced on their own devices and connections. The two can disagree.
Common Causes of Poor Scores
- Slow LCP: oversized images, slow server response, render-blocking CSS and scripts, or a hero image that loads late
- Poor INP: heavy JavaScript that occupies the browser's main thread, long tasks and bulky third-party widgets such as chat tools and tag managers
- High CLS: images and embeds without set dimensions, late-loading fonts, and banners or ads inserted above existing content
Practical Improvements
Compressing images and serving modern formats, setting width and height attributes so space is reserved, deferring non-essential scripts, using caching and a content delivery network, and limiting the number of third-party tags usually produce the biggest gains. Hosting quality matters too, because a slow server response delays everything that follows.
What Core Web Vitals Do for Rankings
Google has said page experience is one of many factors, and that relevant, helpful content can outrank a faster page. Treat the vitals as a quality baseline rather than a ranking shortcut. Their stronger effect is on behaviour: slow or unstable pages frustrate visitors, which hurts enquiries and sales regardless of rankings.
How to Check a Page
Enter the address in PageSpeed Insights to see field and lab results together, or open the Core Web Vitals report in Search Console to find groups of pages needing attention. Re-check after making changes, since field data is collected over a rolling 28-day period and improvements take weeks to show.
Performance-focused development is part of how our website development team works. See also responsive design, frontend development and SSL certificates.