What are Core Web Vitals and how do they affect SEO?

Core Web Vitals are Google's user experience metrics, part of the page experience signals:

• LCP (Largest Contentful Paint) — how long until the main content renders. Target under 2.5 seconds. Usually driven by server response time, render-blocking resources, or a large unoptimised hero image.
• INP (Interaction to Next Paint) — responsiveness to user input, which replaced First Input Delay in 2024. Target under 200 milliseconds. Caused by long JavaScript tasks blocking the main thread.
• CLS (Cumulative Layout Shift) — visual stability. Target under 0.1. Caused by images without dimensions, ads or embeds injected above existing content, and web fonts causing reflow.

How much they matter for ranking: they are a real but modest signal, and Google has described them as a tiebreaker between pages of comparable relevance. They will not rescue poor content, and excellent content will still rank with mediocre vitals. Being honest about this is important — many candidates overstate it.

Where the real value lies is conversion. A faster, more stable page produces measurably better engagement and revenue regardless of ranking, and that is the stronger business argument.

Note: Distinguish lab data from field data. PageSpeed Insights lab scores are diagnostic; the Chrome User Experience Report field data from real users is what Google actually uses. Optimising for a lab score while real users on slow connections still struggle is a common trap.