Website speed test

What this measures

Two different things, deliberately side by side. Field data is the Chrome UX Report: the 75th-percentile Largest Contentful Paint, Interaction to Next Paint, and Cumulative Layout Shift experienced by real Chrome users on this page over the last 28 days. It is the same dataset search ranking systems reference, and it only exists for pages with enough traffic. Lab data is a single Lighthouse run on an emulated mid-range phone over a throttled connection — reproducible, but a simulation.

The documented “good” thresholds are 2.5 s for LCP, 200 ms for INP, and 0.1 for CLS, each assessed at the 75th percentile of real page loads. A page can score well in the lab and still fail in the field; when the two disagree, the field wins — users are the measurement that matters.

How to read the result

Start with field LCP: if it exceeds 2.5 s, the largest element is arriving late — typically server response time, render-blocking resources, or an unoptimized hero image. A high INP points to long tasks on the main thread. Field CLS above 0.1 almost always traces to images without dimensions or late-injected content. The lab TTFB row isolates the server’s share of the delay. For the mechanics behind each metric, see Core Web Vitals: how the metrics work.