PageSpeed Insights
PageSpeed Insights (PSI) is a free Google tool that reports how fast and stable a web page is on mobile and desktop. It combines real-user data from the Chrome UX Report with a lab test run by Lighthouse, then suggests ways to improve.
Field data vs lab data
The most important thing to understand about PSI is that it shows two different kinds of data.
- Field data comes from the Chrome UX Report: real visits from Chrome users over the previous 28 days. This is the data Google uses when it assesses page experience. It only appears if the page or site has enough traffic.
- Lab data comes from a single Lighthouse test run on Google’s servers with a simulated device and connection. It is useful for debugging but can vary from one run to the next.
If the two disagree, trust the field data for judging real-world performance and use the lab data to find causes.
The Core Web Vitals assessment
At the top of the report, PSI says whether the page passes or fails the Core Web Vitals assessment. A pass means the 75th percentile of visits met the good threshold for all three metrics:
| Metric | Good | Poor |
|---|---|---|
| Largest Contentful Paint (LCP) | 2.5 seconds or less | Over 4 seconds |
| Interaction to Next Paint (INP) | 200 ms or less | Over 500 ms |
| Cumulative Layout Shift (CLS) | 0.1 or less | Over 0.25 |
PSI shows data for the specific URL where available and for the whole origin (the entire site) as well.
The performance score
The 0 to 100 score comes from the lab test only. Scores of 90 and above are rated good, 50 to 89 need improvement and below 50 are poor. The score is a helpful summary, but it is not a ranking factor in itself. A page can score 60 and still pass Core Web Vitals in the field, or score 95 and fail.
How to use PSI effectively
- Test your key templates: homepage, a category page, a product or service page and a blog post.
- Check mobile first, as most sites get more mobile visits and mobile results are usually weaker.
- Read the field data and note which metric fails.
- Scroll to Diagnostics and Opportunities to see suggested fixes, such as reducing unused JavaScript or resizing images.
- Fix the biggest issues, then retest. Remember that field data takes up to 28 days to reflect changes.
Common mistakes
- Chasing a perfect 100 score instead of improving real-user metrics.
- Testing only the homepage.
- Expecting immediate field data changes after a fix.
- Treating every suggestion as essential: prioritise those that affect Largest Contentful Paint and other failing metrics.
Key Takeaways
- PageSpeed Insights combines real-user field data with a Lighthouse lab test.
- Field data from CrUX is what Google uses to assess Core Web Vitals.
- The 0 to 100 performance score comes from lab data and is not a ranking factor itself.
- Test several page templates and check mobile results first.
- Field data reflects a rolling 28 days, so improvements take time to show.
Related Terms
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