Core Web Vitals are a set of three specific metrics defined by Google to measure real-world page experience: Largest Contentful Paint (LCP, loading performance), Interaction to Next Paint (INP, responsiveness), and Cumulative Layout Shift (CLS, visual stability).
Each metric targets a distinct kind of frustration: LCP measures how long the largest visible element takes to render, INP measures the delay between a user's interaction and the page visually responding, and CLS measures how much visible content unexpectedly shifts position while loading. Google publishes 'good,' 'needs improvement,' and 'poor' thresholds for each metric, and while Core Web Vitals are only one of many ranking factors, INP itself replaced an earlier metric, First Input Delay, in March 2024, reflecting that even these standardized metrics evolve.
An ecommerce site where the main product image takes four seconds to fully render would likely score poorly on LCP, while a page where an ad loads late and pushes content down unexpectedly would score poorly on CLS, even if overall loading time seems otherwise reasonable.
Interaction to Next Paint (INP) officially replaced FID as a Core Web Vital in March 2024, because INP measures responsiveness across a user's full visit rather than just the very first interaction.
They're one input among many ranking factors Google considers, described as part of its page experience signals — strong Core Web Vitals scores alone don't guarantee high rankings, and content relevance remains a much larger factor.
Google provides several tools for this, including PageSpeed Insights and the Chrome User Experience Report, which report both lab-based test data and real-world field data collected from actual Chrome users visiting the site.