What makes a page slow
Most slow pages are slow for a few reasons: the server takes long to answer (time to first byte), the HTML and its files are heavy, files are not compressed or cached, and scripts and stylesheets in the head block the page from drawing.
Google's Core Web Vitals measure the result for real visitors: Largest Contentful Paint (aim under 2.5 seconds), Interaction to Next Paint (under 200 ms) and Cumulative Layout Shift (under 0.1).
Questions
Is this the same as PageSpeed Insights?
It measures the server side and the page's makeup directly. For lab scores from Lighthouse, the result links to PageSpeed Insights for the same URL.
What is a good server response time?
Under about 800 ms for the first byte is good; under 200 ms is excellent.
Why does my score change between runs?
Networks and servers vary. Run it a few times and look at the trend.
Where is the test run from?
From Cloudflare's network, close to where the request is made.