Definition
Core Web Vitals is a set of three Google-defined metrics for the perceived speed and stability of a web page: Largest Contentful Paint (LCP), Interaction to Next Paint (INP) and Cumulative Layout Shift (CLS). They measure, respectively, how quickly the main content becomes visible, how quickly the page responds to interaction, and how much the layout shifts while loading.
The thresholds are concrete. LCP is good below 2.5 seconds and poor above 4 seconds. INP is good below 200 milliseconds and poor above 500. CLS is good below 0.1 and poor above 0.25. Google assesses at the 75th percentile of real visitors: a page scores 'good' if three quarters of visits stay under the threshold. INP replaced the older First Input Delay metric in March 2024, because that one only measured the first interaction.
The numbers come from two sources. Field data (CrUX, the Chrome User Experience Report) measures real Chrome users and is what Google uses for ranking; you see it in Search Console under Core Web Vitals and in PageSpeed Insights. Lab data (Lighthouse) simulates a visit on a fixed device and is useful for finding causes, but does not count for ranking. A page can score well in the lab and poorly in the field, for example because of slow mobile networks.
Core Web Vitals are part of Google's page experience signals and are a light ranking factor: they tip the balance between pages that are otherwise equivalent, but do not compensate for weak content. The bigger effect is on conversion: slower pages lose visitors before they have seen anything. For AI crawlers, which often use short timeouts and do not execute JavaScript, the server response (TTFB) that precedes LCP matters as well.
The most common causes are well known: oversized or unoptimised images above the fold (LCP), heavy scripts from trackers and chat widgets blocking the main thread (INP), and images without dimensions or late-loading banners (CLS). Traze measures Core Web Vitals per published page and flags in the technical scan which pages exceed the thresholds.