# PageSpeed Insights

Category: [Performance & Core Web Vitals](https://seo-geo-glossary.com/en/glossary/category/performance-core-web-vitals/) › [Basics & measurement](https://seo-geo-glossary.com/en/glossary/category/performance-core-web-vitals/basics-measurement/) · Also known as: PSI

**PageSpeed Insights (PSI)** is a free Google tool that analyzes the performance of a web page on mobile and desktop and suggests improvements. A report combines two kinds of data: field data from real Chrome users and lab data from a Lighthouse test. It is available at pagespeed.web.dev and through an API.

## What is in a report?

- **Field data** comes from the Chrome User Experience Report (CrUX). Google describes it as real users' experiences over the previous 28-day collection period for First Contentful Paint (FCP), Interaction to Next Paint (INP), Largest Contentful Paint (LCP) and Cumulative Layout Shift (CLS), plus an experimental Time to First Byte (TTFB). The data is updated daily.
- **Lab data** comes from Lighthouse. The page is loaded in a controlled environment with an emulated mid-tier mobile device on mobile networks, or a desktop setup on a wired connection. The report adds lab-only metrics such as Speed Index and Total Blocking Time.

Field data is shown for the URL when enough samples exist, otherwise for the whole origin. If the origin does not have enough samples either, no field data appears.

## How are scores shown?

Lab results get a performance score from 0 to 100: 90 and above is good, 50 to 89 needs improvement, below 50 is poor. Field metrics are rated against the thresholds of each metric.

## Why do lab and field data differ?

Lab data comes from one simulated load and field data from many real visits on many devices and connections. A fast lab result can go with weaker field data, and the reverse can happen.

## What does Google say about ranking?

Google's page on Core Web Vitals says they align with what its core ranking systems seek to reward. Good scores in a tool do not guarantee top rankings. Some third-party sources call PageSpeed Insights itself a ranking factor.

## Example

A report for a product page shows LCP "needs improvement" in field data from the last 28 days, while the lab performance score is 94. The field result reflects slower devices and networks than the lab run. Both are useful, and the field data is the basis for judging Core Web Vitals.

## At a glance

- **Metrics:** FCP, LCP, INP, CLS (field data); TTFB as an experimental metric
- **Measures:** User experience of one URL on mobile and desktop
- **Assessment:** Passes Core Web Vitals when LCP, INP and CLS are “good” at the 75th percentile; performance score of 90 or above is good
- **Data source:** Field data: Chrome User Experience Report, previous 28 days; lab data: Lighthouse
- **Docs:** [Google: About PageSpeed Insights](https://developers.google.com/speed/docs/insights/v5/about)

Last checked: October 8, 2026

## Thresholds

- **LCP** (Largest Contentful Paint): good ≤ 2.5 s, needs improvement ≤ 4 s, poor > 4 s ([Source](https://web.dev/articles/lcp))
- **INP** (Interaction to Next Paint): good ≤ 200 ms, needs improvement ≤ 500 ms, poor > 500 ms ([Source](https://web.dev/articles/inp))
- **CLS** (Cumulative Layout Shift): good ≤ 0.1, needs improvement ≤ 0.25, poor > 0.25 ([Source](https://web.dev/articles/cls))

Assessed at the 75th percentile of page loads, mobile and desktop separately.

## Thresholds

- **FCP** (First Contentful Paint): good ≤ 1.8 s, needs improvement ≤ 3 s, poor > 3 s ([Source](https://web.dev/articles/fcp))

Assessed at the 75th percentile of page loads, mobile and desktop separately.

## Web Vitals metrics compared

| Metric | Measures | Core Web Vital? | Field or lab | “Good” per web.dev |
|---|---|---|---|---|
| LCP (Largest Contentful Paint) | Render time of the largest image, text block or video visible in the viewport, from the start of navigation | yes (loading) | field and lab | 2.5 s or less |
| INP (Interaction to Next Paint) | Latency of interactions across the whole visit until the next paint; one value that all (or nearly all) interactions fall below | yes (interactivity), since 2024-03-12 | field; in the lab only with simulated interactions, Lighthouse does not measure INP | 200 ms or less |
| CLS (Cumulative Layout Shift) | Largest burst of unexpected layout shifts during the entire lifecycle of the page | yes (visual stability) | field and lab | 0.1 or less |
| TTFB (Time to First Byte) | Time from the start of navigation until the first byte of the response arrives | no (helps diagnose LCP) | field and lab | 0.8 s or less |
| FCP (First Contentful Paint) | Time from the start of navigation until any part of the page's content is rendered | no (helps diagnose LCP) | field and lab | 1.8 s or less |
| TBT (Total Blocking Time) | Total time after FCP during which the main thread was blocked long enough to prevent input responsiveness | no (lab proxy for INP) | lab; web.dev does not recommend field measurement | less than 200 ms on average mobile hardware |
| Speed Index | How quickly content is visually displayed during page load (from a video of the load) | no | lab (Lighthouse) | Threshold according to Lighthouse: mobile up to 3.4 s “fast” (web.dev gives no value) |
| FID (First Input Delay), historical | Time from the first interaction until the browser can begin processing event handlers | until 2024-03-12, replaced by INP | field only | 100 ms or less |

Sources: [web.dev: Web Vitals](https://web.dev/articles/vitals); [web.dev: Largest Contentful Paint (LCP)](https://web.dev/articles/lcp); [web.dev: Interaction to Next Paint (INP)](https://web.dev/articles/inp); [web.dev: Cumulative Layout Shift (CLS)](https://web.dev/articles/cls); [web.dev: Time to First Byte (TTFB)](https://web.dev/articles/ttfb); [web.dev: First Contentful Paint (FCP)](https://web.dev/articles/fcp); [web.dev: Total Blocking Time (TBT)](https://web.dev/articles/tbt); [web.dev: First Input Delay (FID)](https://web.dev/articles/fid); [web.dev Blog: Interaction to Next Paint is officially a Core Web Vital (2024-03-12)](https://web.dev/blog/inp-cwv-launch); [Chrome for Developers: Lighthouse – Speed Index](https://developer.chrome.com/docs/lighthouse/performance/speed-index). Last checked: October 8, 2026

## Not to be confused with

- [Lighthouse](https://seo-geo-glossary.com/en/glossary/lighthouse/index.md): is the software that simulates a page load and supplies the lab data. Field data from real users comes from the Chrome User Experience Report.

## Related terms

- [Page speed](https://seo-geo-glossary.com/en/glossary/page-speed/index.md)
- [Lighthouse](https://seo-geo-glossary.com/en/glossary/lighthouse/index.md)
- [Core Web Vitals](https://seo-geo-glossary.com/en/glossary/core-web-vitals/index.md)
- [Chrome UX Report (CrUX)](https://seo-geo-glossary.com/en/glossary/chrome-ux-report/index.md)
- [Field data vs. lab data](https://seo-geo-glossary.com/en/glossary/field-data-vs-lab-data/index.md)

## Sources

- [About PageSpeed Insights – Google for Developers](https://developers.google.com/speed/docs/insights/v5/about)
- [Get started with the PageSpeed Insights API – Google for Developers](https://developers.google.com/speed/docs/insights/v5/get-started)
- [Core Web Vitals and Google Search results – Google Search Central](https://developers.google.com/search/docs/appearance/core-web-vitals)
- [Web Vitals – web.dev](https://web.dev/articles/vitals)

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Last updated: October 8, 2026
URL: https://seo-geo-glossary.com/en/glossary/pagespeed-insights/