Core Web Vitals

Core Web Vitals are a set of three page-experience metrics Google defined to measure loading speed, responsiveness and visual stability from a real user's perspective: Largest Contentful Paint, how long the main content takes to load; Interaction to Next Paint, how responsive the page feels to input, which replaced First Input Delay as the official responsiveness metric in March 2024; and Cumulative Layout Shift, how much visible content jumps around unexpectedly while loading.

Core Web Vitals differ from a general page load time figure by being standardized, threshold-based, each metric is rated good, needs improvement or poor, and measured two ways: "field" data drawn from real Chrome users and "lab" data from simulated tests such as Lighthouse, which can disagree with each other since lab tests run under fixed, idealized conditions.

Google has confirmed page experience, including Core Web Vitals, is one of many signals used in search engine results page ranking, generally a minor one relative to content relevance; slow pages also cost more of a site's crawl budget, since each fetch takes longer. The common pitfall is chasing a perfect lab score in a tool like Lighthouse while real-user field data still shows a poor experience, often reflected in a higher bounce rate, for a meaningful share of visitors, since the two measurement types capture different conditions and can diverge substantially; field data is the more accurate reflection of what visitors actually experience.