Core Web Vitals are a small set of user experience measurements that help explain how quickly a page appears, how responsive it feels, and whether elements unexpectedly move.
Largest Contentful Paint (LCP)
LCP measures when the largest visible content element is rendered. It is influenced by server response, critical images, fonts and render-blocking resources. A common aim for a good user experience is an LCP of 2.5 seconds or less for at least 75% of eligible page visits.
Interaction to Next Paint (INP)
INP captures the responsiveness of interactions such as taps and clicks over a visit. Heavy JavaScript and long main-thread tasks can worsen responsiveness. A good INP is 200 milliseconds or less at the 75th percentile.
Cumulative Layout Shift (CLS)
CLS measures unexpected visual movement. Reserve space for images, embeds and banners, and be careful when injecting content above existing elements. A CLS of 0.1 or less is considered good.
Field data versus lab data
PageSpeed Insights may show both real-user CrUX measurements and lab Lighthouse results. Field measurements may lag behind recent changes and may not be available on low-traffic URLs. Use lab tests to diagnose issues and field data to understand trends in real visitor experience.
A better optimization plan
Measure important page types, identify the largest bottleneck, make one change at a time and retest. Keep a log of changes so improvements are understandable and repeatable.
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