PageSpeed Insights

PageSpeed Insights (PSI) is a Google tool that evaluates the user experience of a page on mobile devices and desktop computers and provides suggestions for improvement. You enter a URL and receive two different kinds of measurements. Anyone who confuses the two kinds quickly draws the wrong conclusions.

Lab data and field data

PSI combines two data sources:

• Lab data comes from a Lighthouse measurement that PSI itself runs under controlled conditions. It is suited to troubleshooting because it is reproducible. Google points out, however, that it may not capture real-world bottlenecks.
• Field data shows how real visitors experienced the page. It comes from the Chrome User Experience Report (CrUX). PSI reports users' experiences over the preceding 28 days. Field data offers fewer metrics than lab data but reflects reality better.

If there is too little field data for a single URL, PSI falls back to the level of the entire domain (origin). You then see an average across all pages of the website, not the value of the page you tested. CrUX only captures Chrome users who have enabled usage statistics reporting and history sync, and only publicly discoverable pages (HTTP 200, without noindex). Chrome on iOS is not included. Google does not state an exact minimum number of visits.

The Core Web Vitals

The three Core Web Vitals are the metrics Google particularly points to. They are considered met if the value falls in the “good” range for at least 75 percent of page views (75th percentile, separately for mobile and desktop).

• Largest Contentful Paint (LCP) measures when the largest visible image, video or text block has been rendered. Good: at most 2.5 seconds. Poor: over 4.0 seconds.
• Interaction to Next Paint (INP) measures the response time to all clicks, taps and keyboard input during the visit. Good: at most 200 milliseconds. Poor: over 500 milliseconds.
• Cumulative Layout Shift (CLS) measures unexpected layout shifts. Good: at most 0.1. Poor: over 0.25.

INP replaced the earlier metric First Input Delay (FID). Google declared INP a stable Core Web Vitals metric on March 12, 2024. FID only looked at the first interaction, INP at all interactions of a visit.

The performance score

The large number from 0 to 100 at the top of the report is the Lighthouse performance score and is therefore a pure lab value. From 90 it counts as good, from 50 to 89 as needing improvement, and below that as poor. The score and the Core Web Vitals assessment of the field data can differ significantly because they are measured differently. Besides performance, PSI also assesses accessibility, best practices and SEO via Lighthouse.

How to read the report

• Start with the field data section. It shows whether real visitors experience good values, and it is decisive for the Core Web Vitals.
• Check whether the field data refers to the URL or only to the origin.
• Use the lab data and the recommendations to find causes, such as a hero image that is too large for LCP.
• Measure mobile and desktop separately, because PSI and the Core Web Vitals assessment distinguish both device classes.
• Change something, and then wait: because of the 28-day period, improvements only gradually show up in the field data.

Limits and typical misinterpretations

• The score is not a ranking value. Google writes that the Core Web Vitals align, together with other aspects of page experience, with what its ranking systems aim to reward. The documentation does not contain a promise that a good value leads to better positions. The 100 points from the lab are therefore not a goal in themselves.
• Lab measurements fluctuate from run to run. Individual outliers say little.
• If field data is missing, that is not an error but often a consequence of too few visitors.

Access

PSI runs in the browser at pagespeed.web.dev, without signing in. For automated measurements there is the PageSpeed Insights API. The development of entire groups of pages is shown by the “Core Web Vitals” report in Google Search Console.

Sources

• Google: PageSpeed Insights, basics
• web.dev: Core Web Vitals
• web.dev: Largest Contentful Paint
• web.dev: Interaction to Next Paint
• web.dev: Cumulative Layout Shift
• web.dev: INP becomes a stable Core Web Vitals metric
• Chrome for Developers: CrUX methodology
• Google Search Central: Core Web Vitals