PageSpeed Insights is a free Google tool that scores a URL out of 100 for both mobile and desktop, combining lab data (a simulated test run, similar to what Lighthouse reports) with real-world field data pulled from the Chrome User Experience Report when enough traffic data exists for that page.
That combination is what makes it more useful than Lighthouse alone: the field data reflects how the page actually performs for real visitors on real devices and connections, which is also the data Google's own ranking systems draw from for Core Web Vitals, while the lab data gives a controlled, reproducible baseline for testing whether a specific fix actually helped. A high lab score with weak field data usually means the test environment doesn't reflect real visitor conditions, slower phones, patchy connections, that the lab test didn't account for. Alongside the score, the report lists specific opportunities and diagnostics, ranked roughly by potential impact, which makes it a reasonable starting checklist for where to focus optimization effort first.
