Technical SEO
How to Read a PageSpeed Insights Report (Field vs Lab, Scores, TTFB and Third-Party Scripts)
PageSpeed Insights shows two different kinds of data. Most confusion comes from mixing them up.
PageSpeed Insights reports field data from real Chrome users (the Chrome User Experience Report, CrUX) over the previous 28 days, and lab data from a simulated Lighthouse test.
Google’s Search documentation recommends achieving good Core Web Vitals, which are measured from field data. The lab score is a diagnostic, not the assessment.
Short answer: In PageSpeed Insights, the top section shows field data from real users over 28 days, including the Core Web Vitals assessment (LCP, INP, CLS at the 75th percentile). The lower section shows a Lighthouse lab test with a 0-100 performance score and diagnostics. Use field data to judge real experience, and lab data to debug. Lab scores vary between runs because of network and hardware conditions.
Key Takeaways
- Field data = real users (CrUX, last 28 days). Lab data = one simulated test.
- Core Web Vitals pass when LCP, INP and CLS are good at the 75th percentile.
- Lighthouse scores vary from run to run.
- No field data? The page may lack enough real-user samples.
- TTFB is a large part of LCP; fix server response early.
Thresholds
| Metric | Good | Needs improvement | Poor |
|---|---|---|---|
| LCP | ≤ 2.5 s | ≤ 4 s | > 4 s |
| INP | ≤ 200 ms | ≤ 500 ms | > 500 ms |
| CLS | ≤ 0.1 | ≤ 0.25 | > 0.25 |
| FCP | ≤ 1.8 s | ≤ 3 s | > 3 s |
| TTFB (experimental) | ≤ 800 ms | ≤ 1.8 s | > 1.8 s |
Field vs lab
| Field (CrUX) | Lab (Lighthouse) | |
|---|---|---|
| Source | Real Chrome users, last 28 days | One simulated load |
| Device | Real mix of devices and networks | Mid-tier mobile device on a mobile network, or desktop |
| Use | Judge real experience; Core Web Vitals | Debug and test fixes |
| INP | Measured | Not measured; Total Blocking Time is a proxy |
Why scores change between runs
Google says variability comes from network availability, client hardware and resource contention. Tests can also run from different data centers (North America, Europe or Asia). Run several tests and look at the trend; rely on field data for decisions.
Time to First Byte
TTFB is how long the browser waits for the first byte of the page. web.dev estimates it often makes up a large share of LCP time. Improve it with good hosting, page caching, a CDN, fewer redirects and efficient server-side code.
Third-party scripts
- Chat widgets, trackers, ad tags and review widgets add network requests and main-thread work.
- Remove scripts you no longer use.
- Load non-essential scripts after the main content, or on interaction.
- Check the Lighthouse diagnostics that list third-party usage.
Fixes for each metric: Core Web Vitals explained.
Frequently asked questions
Is a Lighthouse score of 100 needed for SEO?
No. Google’s documentation focuses on Core Web Vitals from field data, alongside many other signals.
Why does PageSpeed Insights show no field data?
The page or site may not have enough real-user samples in CrUX; PSI then falls back to origin data or shows none.
Lighthouse or PageSpeed Insights?
PageSpeed Insights runs Lighthouse and adds field data from CrUX.
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Sources
- PageSpeed Insights: About PageSpeed Insights
- Google Search Central: Understanding Core Web Vitals and Google search results
- web.dev: Optimize Largest Contentful Paint
Last reviewed October 14, 2026.
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