What is Core Web Vitals?

Core Web Vitals are a standardized subset of Web Vitals that Google uses to measure a page's user experience in the field. They consist of three specific metrics: Largest Contentful Paint (LCP) measures loading performance by recording the time when the largest content element (image, video, or text block) becomes visible within the viewport. Interaction to Next Paint (INP) assesses a page's responsiveness by measuring the time from a user interaction (click, tap, keypress) to the next paint that reflects the result. Cumulative Layout Shift (CLS) quantifies visual stability by summing the unexpected layout shift scores that occur during the page's lifespan.

Each metric has a threshold that defines a good, needs improvement, or poor user experience. For LCP, the target is under 2.5 seconds. For INP, the target is under 200 milliseconds. For CLS, the target is a score less than 0.1. These thresholds are based on the 75th percentile of page loads, as measured from real user data collected by the Chrome User Experience Report (CrUX). Google uses Core Web Vitals as a ranking signal in its search algorithm, meaning pages that meet the good thresholds may receive a ranking boost over those that do not.

Core Web Vitals sit at the intersection of front-end performance optimization and content delivery. They are influenced by server response times, resource loading order, JavaScript execution, and third-party scripts. CDNs and caching strategies directly impact LCP by reducing time to first byte (TTFB) and serving optimized assets. INP is improved by breaking up long tasks and deferring non-critical scripts. CLS is mitigated by setting explicit width and height attributes on images and ads, and by reserving space for dynamic content. Monitoring Core Web Vitals requires tools like Lighthouse, PageSpeed Insights, and the Web Vitals JavaScript library, all of which report field data or lab data against the same thresholds.