Google uses page speed in mobile search and ad ranking, and their Lighthouse tool provides the PageSpeed Insights (PSI) score to help you measure and boost your page’s performance. Let’s look under the hood at how it works.
The PSI score is between 0 and 100 and is generated from a Lighthouse run. Google considers a speed score of 90–100 as fast, 50–89 as average, and 0–49 as slow. You can think of PSI as a hosted Lighthouse web interface (and API).
The Lighthouse scoring documentation explains that the performance score is determined using the following estimated metrics:
I built a HTML Flamegraph tool that help you visualise and diagnose where your HTML payload bytes come from:
See some examples below: