What Google Lighthouse Is and How PerfoMetrix Uses It
Google Lighthouse is an open-source tool from the Chrome team that loads a web page in a controlled browser and reports how fast it is, how accessible it is, and whether it follows good practice for search and security. It is the engine behind PageSpeed Insights, the Lighthouse panel in Chrome DevTools, and many monitoring products, including the performance section of every PerfoMetrix report.
Why it matters
A shared yardstick. Developers, agencies and site owners can run the same test and talk about the same numbers. When your developer opens DevTools and runs Lighthouse, they see the metrics our report shows.
It explains, not just measures. Next to every metric, Lighthouse lists what slowed the page down: render-blocking files, oversized images, long JavaScript tasks, layout shifts. Our performance checks come from those findings.
What Lighthouse measures
Lighthouse has four main report categories:
- Performance: how quickly the page shows content and becomes usable.
- Accessibility: automated checks such as colour contrast, labels and ARIA use.
- Best practices: HTTPS, console errors, deprecated APIs, image aspect ratios.
- SEO: crawlability, titles, descriptions, link text and similar basics.
Newer versions add an agentic browsing category with checks such as llms.txt, aimed at AI agents that read pages.
How the performance score is built
The performance score (0–100) is a weighted mix of five lab metrics:
| Metric | What it measures | Weight |
|---|---|---|
| First Contentful Paint (FCP) | When the first text or image appears | 10% |
| Speed Index | How quickly the visible page fills in | 10% |
| Largest Contentful Paint (LCP) | When the main content appears | 25% |
| Total Blocking Time (TBT) | How long scripts block the page from responding | 30% |
| Cumulative Layout Shift (CLS) | How much the layout jumps while loading | 25% |
Lighthouse colours its own scores in three bands: red from 0 to 49, orange from 50 to 89 and green from 90. PerfoMetrix colours every section score by its letter grade instead (red below 60, orange 60–69, yellow 70–89, green 90 and above), so the colour and the grade always agree.
Lab data and field data
Lighthouse runs a lab test: one page load on a simulated mid-range phone with a throttled network. That makes results comparable, but it is a single moment.
The Chrome UX Report (CrUX) is field data: how real Chrome users experienced the page or site over the last 28 days. Google uses field data for its page experience signals. A PerfoMetrix report shows both: the Lighthouse lab result and, when Google has enough visits for the page or site, the real-user values.
Why scores change from run to run
A lab test measures one load. Server response time, network conditions, ads and other third-party scripts, and A/B tests all vary, so a few points of difference between runs is normal. To judge a change, run several tests or watch the trend over time (a monitor does this for you).
How PerfoMetrix runs Lighthouse
We call Google’s PageSpeed Insights API, which runs the current Lighthouse release on Google’s servers. Every scan runs the mobile test; paid plans also run desktop. The report records the Lighthouse version that produced the numbers, so results stay comparable when Google ships a new release.
Related guides
- Google Lighthouse versions explains what changed from release to release.
- Where our data comes from lists every source a report uses.
- The guides on Largest Contentful Paint, layout shifts and render-blocking resources explain how to fix the main findings.