How to use the Page Speed Test
- Enter the URL and choose Mobile (the default Google uses for indexing) or Desktop.
- Click Test speed. Lighthouse loads the page in a throttled browser — this usually takes 20–60 seconds.
- Check the Core Web Vitals: green is good, amber needs improvement, red is poor. Real-user field data, when shown, is what Google uses for ranking.
- Work through the opportunities from the largest estimated saving down, then test again.
What is a page speed test?
A page speed test loads a URL in a controlled browser and measures how quickly content appears, how soon the page responds to input and how stable the layout is. This tool runs Google Lighthouse through the PageSpeed Insights API — the same engine as Chrome DevTools — and, where Google has enough traffic data for the URL, adds field data from the Chrome User Experience Report (CrUX).
Core Web Vitals and their thresholds
Core Web Vitals are Google's user-experience metrics and part of its page experience signals. Each is assessed at the 75th percentile of page loads:
| Metric | Measures | Good | Poor |
|---|---|---|---|
| LCP — Largest Contentful Paint | When the main content (hero image or headline) has rendered | ≤ 2.5 s | > 4.0 s |
| INP — Interaction to Next Paint | Responsiveness to clicks, taps and key presses | ≤ 200 ms | > 500 ms |
| CLS — Cumulative Layout Shift | Unexpected movement of content while loading | ≤ 0.1 | > 0.25 |
INP replaced First Input Delay in March 2024. Because INP requires real user interactions, a lab test can't measure it; Lighthouse reports Total Blocking Time (TBT) instead, which correlates with it (good ≤ 200 ms). Lighthouse also reports First Contentful Paint (good ≤ 1.8 s) and Speed Index.
Lab data vs. field data
Lab data comes from one simulated load: on mobile, Lighthouse emulates a mid-range phone on a throttled 4G connection, so scores vary from run to run and are usually lower than on your own fast device. It's ideal for debugging. Field data aggregates real Chrome users over the previous 28 days. It is what Google uses in its page experience assessment, but it only exists for pages with enough traffic.
How the performance score is calculated
The 0–100 performance score is a weighted average of lab metrics. In current Lighthouse versions Total Blocking Time carries 30 %, LCP and CLS 25 % each, and FCP and Speed Index 10 % each. 90–100 is good, 50–89 needs improvement, below 50 is poor.
Fixes that help most often
- LCP: compress and correctly size the hero image (try the Image Compressor), serve WebP/AVIF, don't lazy-load it, and reduce server response time.
- TBT / INP: remove unused JavaScript and heavy third-party tags, defer non-critical scripts, and break up long tasks.
- CLS: set width and height on images and embeds, reserve space for ads and cookie banners, and use
font-displaywith matched fallback fonts.
For HTML-level performance checks such as compression, caching headers and render-blocking scripts, run the SEO Checker.
Tools that pair well with the Page Speed Test
- Image Alt Checker — Audit every image on a page in one table: alt text status, file size, width/height attributes, format and lazy loading — with filters and CSV export.
- Security Headers Checker — Grade a site’s HTTP security headers from A+ to F, see which are missing and why they matter, and copy ready-made config snippets for Nginx, Apache, Netlify, Vercel or Cloudflare.
- Website Down Checker — Find out in seconds whether a website is down for everyone or just for you — with status code, response time and troubleshooting tips.
- Meta Tag Checker — See exactly how Google is likely to display your page: title and description measured in pixels, desktop and mobile previews, and every meta tag in the <head>.