In short
Core Web Vitals are a set of three metrics defined by Google to measure real-world user experience on a web page: Largest Contentful Paint (LCP) for loading performance, Interaction to Next Paint (INP) for responsiveness, and Cumulative Layout Shift (CLS) for visual stability. Together they quantify how fast, responsive, and stable a page feels to visitors.
The three Core Web Vitals metrics
Each metric has published thresholds for a 'good' experience, measured at the 75th percentile of page loads:
- Largest Contentful Paint (LCP) measures how long the largest visible element takes to render. Good is 2.5 seconds or less.
- Interaction to Next Paint (INP) measures responsiveness to user interactions across the whole visit. Good is 200 milliseconds or less. INP replaced First Input Delay (FID) as a Core Web Vital in March 2024.
- Cumulative Layout Shift (CLS) measures unexpected movement of page elements. Good is 0.1 or less.
How Core Web Vitals are measured
Core Web Vitals can be assessed two ways. Field data reflects real users and is collected in the Chrome User Experience Report (CrUX); this is the data Google uses to assess a site. Lab data is generated in a controlled test environment and is useful for debugging but does not reflect actual visitors.
Because Google evaluates the 75th percentile of real visits, a page passes only when at least three-quarters of experiences meet the 'good' threshold for each metric.
Why Core Web Vitals matter
Core Web Vitals are part of Google's page experience signals and contribute to ranking, though content relevance remains the dominant factor. Their larger value is user-facing: faster, more stable, more responsive pages reduce bounce, improve conversions, and build trust.
They should be treated as a threshold to clear rather than a score to maximize. A page that meets all three 'good' thresholds gains the page-experience benefit; pushing metrics far beyond 'good' yields diminishing returns.
How to improve Core Web Vitals
Typical fixes by metric:
- LCP: optimize and compress images, serve them in modern formats, use a content delivery network, and remove render-blocking resources.
- INP: break up long JavaScript tasks, defer non-critical scripts, and minimize main-thread work so the page responds quickly to input.
- CLS: set explicit width and height on images and embeds, reserve space for ads, and avoid inserting content above existing content.
Monitoring field data over time — as Novaverb and similar tools do — reveals whether real-user experience is trending toward or away from the 'good' thresholds.