The test runs Google's PageSpeed Insights on your URL as a phone, then gives a mobile score out of 100, five load timings and the fixes that save the most time. For the phone test, Lighthouse, which PageSpeed Insights runs, uses a 1.6 Mbps connection with 150 ms of latency and a processor slowed four times.
Why the mobile score is lower
Most pages score lower on mobile because of that throttling: the same page loads over a slower connection on a slower processor, so every large image and script costs more.
The desktop result beside it is tested on a faster connection. The gap between the two scores shows how much a slow phone connection costs your page.
Core Web Vitals use real visitors
Google's Core Web Vitals do not use this score from one simulated load; they use what real Chrome users experienced over the previous 28 days.
A page counts as good when, at the 75th percentile of those visits, Largest Contentful Paint is 2.5 seconds or less, Interaction to Next Paint is 200 milliseconds or less and Cumulative Layout Shift is 0.1 or less.
This website speed test is free with no signup. SearchBlueprint, who make it, sell SEO planning for websites: which searches each page should answer.