PageSpeed Insights
PageSpeed Insights (PSI) is a free Google tool that reports on the user experience of a web page on mobile and desktop devices. It combines two kinds of data: field data from real Chrome users from the CrUX report, and lab data from a Lighthouse simulation. Based on these, it gives a Core Web Vitals assessment, a performance score from 0 to 100 and optimization suggestions. Field data covers the last 28 days. The tool works in the browser at pagespeed.web.dev and through an API.
What is PageSpeed Insights?
PageSpeed Insights is a Google tool for analyzing the speed and user experience of a page. According to Google's documentation, it reports on the user experience on mobile and desktop devices and suggests how the page can be improved.
PSI's key feature is combining lab and field data in a single report. The documentation explains that lab data helps diagnose issues in a controlled environment. Field data shows the real user experience but covers fewer metrics.
| Feature | Description |
|---|---|
| Creator | |
| Type | web page performance analysis tool |
| Address | pagespeed.web.dev |
| Field data | Chrome User Experience Report (CrUX), last 28 days |
| Lab data | Lighthouse, simulated page load |
| Devices | mobile and desktop versions |
| Programmatic access | PageSpeed Insights API |
What data does PageSpeed Insights show?
PageSpeed Insights shows field data from real users and lab data from a simulation. The two come from different sources and can produce different results.
| Feature | Field data | Lab data |
|---|---|---|
| Source | CrUX report, real Chrome users | Lighthouse, simulation |
| Period | last 28 days | a single test at the time of analysis |
| Conditions | various devices and connections | one device and fixed network conditions |
| Metrics | FCP, LCP, CLS, INP and, experimentally, TTFB | FCP, LCP, Speed Index, CLS, TBT and others |
| Use | assessing the real experience | diagnosing and debugging issues |
Field data requires enough visits. When a page is new or has too little data, PSI shows data for the whole origin. When that data is missing too, the report has no field data. According to the documentation, CrUX only collects data for public, indexable URLs.
Lab data comes from Lighthouse. PSI rates four categories in it: performance, accessibility, best practices and SEO. According to the documentation, the mobile version simulates a mid-tier device on a mobile network, and the desktop version a wired connection. The test runs in a Google data center in North America, Europe or Asia.
The documentation explains that field and lab data can contradict each other. Field data is a record of many users' experiences, while lab data is a simulation on a single device. A good lab score therefore does not automatically mean a good experience for real users.
How does PageSpeed Insights assess Core Web Vitals?
PageSpeed Insights assesses Core Web Vitals based on field data from the CrUX report. Core Web Vitals are three metrics: LCP, INP and CLS.
PSI groups user experiences into three buckets: good, needs improvement and poor. The thresholds follow the Web Vitals initiative.
| Metric | Good | Needs improvement | Poor |
|---|---|---|---|
| LCP (Largest Contentful Paint) | up to 2.5 s | 2.5–4 s | over 4 s |
| INP (Interaction to Next Paint) | up to 200 ms | 200–500 ms | over 500 ms |
| CLS (Cumulative Layout Shift) | up to 0.1 | 0.1–0.25 | over 0.25 |
| FCP (First Contentful Paint) | up to 1.8 s | 1.8–3 s | over 3 s |
| TTFB (experimental) | up to 800 ms | 800–1800 ms | over 1800 ms |
Above the charts, PSI shows the 75th percentile of each metric. According to the documentation, this percentile shows how the page performs for users in the most difficult device and network conditions.
A page or origin passes the Core Web Vitals assessment when the 75th percentile of all three metrics is good. When INP data is missing, good LCP and CLS are enough. When LCP or CLS data is missing, no assessment is possible.
The Lighthouse performance score is reported separately. A score of 90 or above is considered good, 50–89 needs improvement and below 50 is poor.
How do you use PageSpeed Insights and its API?
To use PageSpeed Insights, enter a page address in the browser at pagespeed.web.dev. The report shows separate results for mobile and desktop.
For automation, Google provides the PageSpeed Insights API. According to the documentation, the API returns field data from the CrUX report and lab data from Lighthouse.
| API feature | Description according to Google's documentation |
|---|---|
| Version | v5 |
| Data returned | field data from CrUX and lab data from Lighthouse |
| API key | not required, but recommended for frequent, automated queries |
| Getting a key | the Credentials page in Google Cloud Console |
Field data in PSI differs from the CrUX dataset in BigQuery in how often it is updated. According to the documentation, PSI updates data daily, while BigQuery updates once a month and only at the origin level. Both cover the last 28 days.
Lab results depend on the Lighthouse version. Every report shows it next to the emulated device information. After a Lighthouse update, a page's score can change without any change to the page itself.
What sources is this description based on?
How the tool works, the types of data, metric thresholds and Core Web Vitals assessment rules are described in Google's "About PageSpeed Insights" documentation. Information about the API comes from "Get Started with the PageSpeed Insights API". The relationship between PSI and Lighthouse is described in "Introduction to Lighthouse" on Chrome for Developers.
Frequently asked questions
Q.Why does the PageSpeed Insights score change with every test?
According to Google's documentation, the performance score changes because of measurement variability. Common causes are the availability of network, hardware and resources in the test environment. The test runs in a Google data center whose conditions can vary. Scores also change after Lighthouse updates, so it is worth comparing reports from the same version.
Q.Why doesn't PageSpeed Insights show data from real users?
Real-user data comes from the CrUX report, which needs enough visits. According to the documentation, the URL must be public and indexable. A new page or a page with little traffic may have no data. PSI then shows data for the whole origin, and when that is missing too, only lab data.
Q.What is a good PageSpeed Insights score?
A good performance score is 90 or more. A score of 50–89 needs improvement, and below 50 is poor. Google's documentation notes, however, that a good lab score does not automatically mean a good user experience. The Core Web Vitals assessment from field data matters more, and in it LCP, INP and CLS should all be good.
Q.How is PageSpeed Insights different from Lighthouse?
Lighthouse is a tool for a simulated page audit, while PageSpeed Insights is a Google service that uses it. PSI adds field data from real users from the CrUX report to the Lighthouse results, and shows a Core Web Vitals assessment. Lighthouse can be run locally, for example in Chrome DevTools, while PSI works in the browser and through an API.
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