Core Web Vitals

Core Web Vitals are a set of standardized, user-centric metrics defined by Google to measure the real experience of loading and interacting with a web page. They are part of the broader Web Vitals initiative and influence Google Search ranking, which has made them a central performance target for frontend teams and a common item on product and SEO roadmaps alike.

What It Measures

Three metrics make up Core Web Vitals. Largest Contentful Paint (LCP) measures loading performance, the time until the largest visible content element renders; good is 2.5 seconds or less. Interaction to Next Paint (INP) measures responsiveness, the latency of user interactions across the entire page lifetime; good is 200 milliseconds or less. Cumulative Layout Shift (CLS) measures visual stability, how much content unexpectedly moves during load; good is 0.1 or less. INP replaced First Input Delay (FID) as the responsiveness metric in 2024 because it captures the full interaction, not just the first one.

Methodology

Core Web Vitals are assessed two ways. Field data (real-user monitoring) comes from actual visitors, aggregated in sources like the Chrome User Experience Report (CrUX); thresholds are evaluated at the 75th percentile of users so that a passing site is good for most, not just median, visitors. Lab data comes from synthetic tools such as Lighthouse, which simulate a page load under controlled, throttled conditions for diagnostics and reproducibility. Field data determines whether a site passes; lab data helps reproduce and debug issues before they reach users. The open-source web-vitals JavaScript library lets teams collect these metrics directly from their own users in production.

How to Interpret Results

A URL passes Core Web Vitals when the 75th-percentile field value for all three metrics falls in the good range. Use field data as the source of truth, since it reflects real devices, networks, and usage patterns, and lab data to investigate causes. A poor LCP points to slow servers, render-blocking resources, or large unoptimized images; poor INP points to heavy JavaScript blocking the main thread; poor CLS points to unsized media, late-injected content, or web fonts causing reflow. Lower values are better for all three metrics.

Limitations

Lab tools test one simulated environment and can disagree with field data, which spans many real devices and networks; never optimize lab scores at the expense of real users. The 75th-percentile threshold means some users always experience worse performance even on a passing site. Metrics evolve over time, as the FID-to-INP change showed, so targets and tooling shift and historical comparisons can break. Core Web Vitals capture key experience dimensions but not everything, such as time-to-interactive nuances or error rates; complement them with full real-user monitoring.