Lighthouse

Lighthouse is built into Chrome's developer tools and provides the lower part of the PageSpeed Insights report, the lab test. It loads the page once under fixed conditions, for the mobile check on a simulated mid-range smartphone on a mobile network, and gives each category a score from 0 to 100. Google divides the scores into three bands: 0 to 49 is poor, 50 to 89 needs improvement, 90 and above is good.

The performance score is a weighting of individual metrics. In Lighthouse 10, Total Blocking Time counts for 30 per cent, Largest Contentful Paint and Cumulative Layout Shift for 25 per cent each, First Contentful Paint and Speed Index for 10 per cent each. Total Blocking Time is a lab substitute: how quickly a page responds to real input, measured in the field as Interaction to Next Paint, cannot be captured by a test run without people. Google itself states that the score fluctuates, for example because of changes in internet traffic routing, browser extensions, antivirus software, changing ads or A/B tests.

The most important limit is where the number comes from. Lighthouse measures one session in the lab, while the field data in the same report comes from real visitors over the last 28 days, and only that shows how the page performs for its audience. The accessibility score has a limit too: it is a weighted average of automatically testable audits, and the manual audits do not count. A score of 100 therefore means the automated audits have passed, not that the page is accessible.