Core Web Vitals are a set of metrics Google uses to measure real-world user experience. They directly impact search rankings and are part of Google's page experience signal.
LCP measures how long it takes for the largest visible element on the page to load. This is usually a hero image, heading, or video. Google's target is under 2.5 seconds.
Slow LCP is typically caused by slow server response times, render-blocking resources, or unoptimised images. Fixing these also improves general page speed.
FID measures the time between a user first interacting with your page (clicking a button, tapping a link) and the browser responding. Target: under 100 milliseconds.
High FID is caused by heavy JavaScript that blocks the main thread. Break up long tasks, defer non-essential scripts, and use web workers for heavy computations.
Note: FID will be replaced by INP (Interaction to Next Paint) as of March 2024. INP measures all interactions, not just the first one.
CLS measures unexpected layout shifts during page load. You have experienced this when text jumps down as images load or when you try to tap a link and something else moves under your finger.
Good CLS is under 0.1. Common causes: images without dimensions, ads and embeds without reserved space, late-loading fonts, and dynamic content injected above existing content.
Google Search Console's Core Web Vitals report shows how your pages perform based on real user data (Chrome UX Report). Lighthouse provides diagnostic information for development. PageSpeed Insights combines both.
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