PageSpeed Insights

PageSpeed Insights and Lighthouse are often treated as the same tool because they share the same scoring engine, but they measure two different things. Lighthouse, run locally via Chrome DevTools, the CLI, or CI, produces lab data: a single, controlled, reproducible test on your machine with the throttling and device emulation you choose. PageSpeed Insights runs that same Lighthouse engine on Google's own servers, then layers real-user field data from the Chrome UX Report (CrUX) on top, a rolling 28-day window of how actual visitors on real devices and networks experienced the page, reported at the 75th percentile.

That field data is the part that actually matters for rankings. Google has confirmed that Core Web Vitals, the ranking signal, comes from CrUX field data, not from Lighthouse's lab score. A page can score 95 in a local Lighthouse run and still show "Poor" Largest Contentful Paint in Search Console's Core Web Vitals report, because real visitors on slower connections and older devices are having a worse experience than the lab test simulated. This is also why PSI scores fluctuate between runs in a way that can be confusing: shared server infrastructure, varying network routing, and the CrUX rolling window all introduce variance that a fixed local Lighthouse run doesn't have.

Used well, the two tools are complementary rather than competing. PSI's field data tells you whether real users are actually having a bad experience and whether that's likely affecting rankings. Lighthouse, run locally or in CI, gives you a controlled way to isolate a specific fix, verify it worked, and catch regressions before they ship, without waiting weeks for the CrUX window to update. Neither replaces a full technical crawler; they measure one page's speed and Core Web Vitals, not site-wide crawlability or indexation issues.