How to use the website speed checker
- Enter the page URL and choose mobile or desktop.
- Select Test speed and wait up to a minute while Lighthouse runs.
- Read field data first: it reflects real Chrome users over 28 days, when Google has enough data.
- Use the lab metrics and top opportunities to find what to fix, then retest.
Worked example
When the API quota is used up
Testing https://example.com without an API key on a busy day returned the message "The PageSpeed Insights API quota has been reached" with a link to run the same test on pagespeed.web.dev. No numbers are shown unless Google returns them.
How it works
The URL is validated, then sent to the Google PageSpeed Insights API v5 (performance category) with a 60 second timeout. Lab metrics (performance score, FCP, LCP, TBT, CLS and Speed Index) come from a single Lighthouse run. Field data comes from the Chrome UX Report (75th percentile of real page loads over 28 days) and is shown only when the API returns it, for the URL or its whole origin. Opportunities are the failing audits reported by Lighthouse, sorted by estimated savings.
Assumptions
- All numbers come from Google’s API response. Nothing is estimated by this site.
- Lab results vary between runs and depend on Google’s test location and throttling.
Frequently asked questions
What is the difference between lab and field data?
Lab data is one simulated Lighthouse run, useful for debugging. Field data is what real Chrome users experienced over the last 28 days, and it is what Core Web Vitals assessments use.
Why is field data missing?
The Chrome UX Report only includes URLs and origins with enough real-user traffic. New or low-traffic pages often have lab data only.
Why did the test fail with a quota message?
The free PageSpeed Insights API has a shared daily quota. Site owners can add a PAGESPEED_API_KEY on the server to raise it. You can always run the test directly on pagespeed.web.dev.
Is the performance score a ranking score?
No. It is Lighthouse’s own summary of lab metrics. Google uses field Core Web Vitals as one of many page experience signals.
Limitations
- Depends on the Google PageSpeed Insights API being available and within quota.
- Tests one URL per run and cannot test pages behind logins.
- Limited to 10 tests per visitor every 10 minutes.