PageSpeed Insights (PSI) is a free tool by Google that measures the loading time of your website. Website performance impacts user experience and is a Google ranking factor.
PSI reports both field data from real users and lab data collected on Google servers. The real user data is collected as part of the Chrome User Experience Report (CrUX). The lab data is based on the free open-source Lighthouse tool.
In addition to testing performance Lighthouse also assesses SEO, accessibility, and general website best practices.
When running a speed test on PSI Google automatically tests your site on desktop and mobile devices.
It then gives you a detailed report that includes ways you can improve.