PageSpeed Insights is a free Google tool that reports on a URL's performance in two ways. The top section shows field data from the Chrome UX Report: how real Chrome users experienced the page and the whole origin over the last 28 days, including a Core Web Vitals assessment. The lower section runs a Lighthouse lab test and lists diagnostics and opportunities.
Field data section
If the URL has enough traffic, PSI shows its own field data; otherwise it may fall back to origin-level data for the whole site, or show nothing. The Core Web Vitals assessment passes when LCP, INP and CLS are all good at the 75th percentile. This is the same data source Search Console's Core Web Vitals report uses, so it is the closest view of what Google measures.
Lab data section
The Lighthouse run happens on Google's servers with simulated mobile or desktop settings. It is repeatable and gives detailed advice, but reflects one simulated load. Use it to find causes, and use field data to judge whether real users are affected.
The API
The PageSpeed Insights API returns the same field and lab data as JSON, which makes it easy to monitor key URLs over time or across templates. It requires an API key for regular use.
GET https://www.googleapis.com/pagespeedonline/v5/runPagespeed
?url=https://example.com/pricing&strategy=mobile&key=YOUR_KEYExample
Lab says fine, field says poor
Thistlecrane's pricing page scores well in the PSI lab section, but the field section shows a poor INP. The lab test never clicks the monthly-to-annual toggle, which recalculates every plan on the main thread. Profiling that interaction in DevTools finds the slow handler that the lab score missed.
Common mistakes
Quoting only the lab score
The 0 to 100 score is from Lighthouse. Google's assessment of real experience is the field section.
Testing the homepage only
Templates differ. Test a representative URL for each page type: blog, docs, pricing and feature pages.
Questions
CrUX only publishes data for pages and origins with enough eligible Chrome traffic. New or low-traffic pages may have none.
