PageSpeed Insights (PSI) is a Google tool that analyzes web page performance using Lighthouse. It provides a performance score (0-100), Core Web Vitals metrics, and actionable optimization recommendations for both mobile and desktop.

PageSpeed Insights runs Lighthouse audits on Google's servers and adds real user data from CrUX (Chrome User Experience Report). Lighthouse in Chrome DevTools runs locally. PSI also provides field data showing how real users experience your page.

PSI applies CPU throttling (4x slowdown) and network throttling for mobile tests, simulating a mid-range phone on 4G. Desktop tests use minimal throttling. This means JavaScript-heavy pages score significantly lower on mobile.

Focus on the highest-impact metrics: reduce Total Blocking Time (30% weight) by code-splitting JavaScript, optimize Largest Contentful Paint (25%) by preloading key resources, and minimize Cumulative Layout Shift (25%) by setting image dimensions.