PageSpeed Insights (PSI) is a free Google tool that analyses how fast a web page loads and how good the user experience is on mobile and desktop. It combines real-user data from the Chrome User Experience Report with lab tests from Lighthouse, then offers recommendations to improve performance.
Field Data vs Lab Data
| — | — | — |
|---|---|---|
| Field data | Real Chrome users over 28 days | Shows actual user experience and Core Web Vitals status |
Field data may not appear for low-traffic pages; in that case, rely on lab data and origin-level field data.
Core Web Vitals in PSI
| — | — | — |
|---|---|---|
| Largest Contentful Paint (LCP) | ≤ 2.5 s | Loading of main content |
| Interaction to Next Paint (INP) | ≤ 200 ms | Responsiveness |
| Cumulative Layout Shift (CLS) | ≤ 0.1 | Visual stability |
PSI shows whether a page passes the Core Web Vitals assessment based on field data.
Lab Metrics
- First Contentful Paint
- Largest Contentful Paint
- Total Blocking Time (a lab proxy related to responsiveness)
- Cumulative Layout Shift
- Speed Index
Understanding the Performance Score
The 0–100 score is calculated from lab metrics. Scores vary between runs due to network and server conditions. Aim for steady improvement rather than a perfect 100.
Common Recommendations
| — | — | — |
|---|---|---|
| Serve images in next-gen formats | Use WebP or AVIF | Image optimisation plugin |
| Eliminate render-blocking resources | CSS/JS delays rendering | Defer scripts, inline critical CSS |
| Reduce unused JavaScript | Scripts loaded but not used | Remove or delay plugins and third-party code |
| Reduce server response time | Slow backend | Better hosting, caching |
| Avoid large layout shifts | Elements move while loading | Set image dimensions, reserve ad space |
| Minimise main-thread work | Heavy scripts | Simplify page, reduce third-party scripts |
How to Use PSI Effectively
- Test key templates: homepage, blog post, category page, product page
- Test both mobile and desktop
- Record results before changes
- Fix the biggest issues first
- Retest after each change
- Monitor field data over the following weeks
Mobile vs Desktop Scores
Mobile scores are usually lower because PSI simulates a mid-range phone on a slower network. Since many visitors use phones, prioritise mobile improvements.
Common Mistakes
- Chasing 100 scores at the expense of functionality
- Ignoring field data
- Testing only the homepage
- Making many changes at once without measuring
Frequently Asked Questions
Is the PSI score a ranking factor?
No. Core Web Vitals from real users are part of page experience signals, but the lab score itself is not.
Why does my score change every test?
Lab tests vary with network and server conditions.
Why is there no field data?
The page may not have enough real Chrome user visits.
What is a good score?
Aim for green Core Web Vitals and steady lab improvements; 90+ is excellent but not required.
Conclusion
PageSpeed Insights helps you understand and improve real user experience. Focus on field Core Web Vitals, use lab diagnostics to find fixes, prioritise mobile and measure changes carefully. Faster pages benefit visitors, conversions and SEO.
