What It Is
Lighthouse is built into the Chrome browser and powers Google's free website at PageSpeed Insights. Point it at any page and, in about a minute, it returns four scores from 0 to 100:
- Performance: how quickly the page loads and becomes usable.
- Accessibility: whether people using screen readers or other assistive tools can use the page.
- Best Practices: security and modern web standards.
- SEO: the technical basics a search engine needs, like a real title, a description, links it can follow, and a robots file that is not blocking it.
Each score comes with a list of what passed, what failed, and why.
How It Works
Lighthouse loads the page once, on a simulated phone over a throttled connection, and grades what it sees. That makes it a lab test. It is excellent for finding causes, because it names the exact image or script slowing a page down. It is also a single run, so the performance score moves between runs, sometimes by several points, on a page that has not changed at all. When PageSpeed Insights has enough real visits to report, it shows that field data above the Lighthouse scores, and the field data is the one Google uses in Search.
Why It Matters for Being Found
The SEO score is the useful floor. It says nothing about how competitive your market is, and that is the point: every business can score 100, because every check on the list is within your control. Scoring less means a search engine is tripping over something that is simple to fix. The performance score is a good map of what to fix for speed, and Core Web Vitals is how you learn whether the fix reached real customers.