Core Web Vitals

Core Web Vitals (CWV) are Google's set of user-experience metrics designed to quantify how a page feels to load and interact with. The metrics became a ranking signal in 2021 as part of the Page Experience update and have been refined since.

The three current Core Web Vitals are: Largest Contentful Paint (LCP) (Largest Contentful Paint, measuring loading performance), Interaction to Next Paint (INP) (Interaction to Next Paint, measuring responsiveness — replaced First Input Delay in March 2024), and Cumulative Layout Shift (CLS) (Cumulative Layout Shift, measuring visual stability). Each has Good, Needs Improvement, and Poor thresholds.

CWV are evaluated at the 75th percentile of page loads across real users, not lab tests. This is critical — synthetic Lighthouse scores can be excellent while real-user data is poor, because lab tests run on simulated conditions that may not reflect actual user devices and networks. The data source Google uses for ranking is the Chrome User Experience Report (CrUX), aggregated from real Chrome users.

CWV are a ranking signal, but they are a tie-breaker rather than a primary factor. Pages with stronger content and links typically outrank pages with better CWV but weaker substance. The right framing: CWV becomes meaningful when you are competing against pages of similar topical strength, and as a UX issue independent of rankings (poor CWV correlates strongly with higher bounce and lower conversion).

The most common CWV failures: LCP failures from large unoptimised hero images or render-blocking JavaScript; INP failures from heavy third-party scripts blocking the main thread; CLS failures from images without dimensions, web fonts swapping in, and dynamically injected ad slots.

Track CWV through three sources: PageSpeed Insights for per-URL diagnostics, Search Console's Core Web Vitals report for site-wide CrUX data segmented by URL group and device, and your own RUM (real-user monitoring) if you operate at scale. Field data should drive decisions; lab data should drive root-cause analysis.

Time to First Byte (TTFB) (Time to First Byte) is not officially a CWV metric but is treated as a foundational input — slow server response makes every downstream metric worse. Improving TTFB through caching, CDN, and back-end performance is usually the highest-leverage starting point.