Google PageSpeed Insights is the starting point for almost every performance investigation, and for good reason. It gives you two distinct views of your page: Lab Data, which is a simulated test run in a controlled environment, and Field Data, which comes from real Chrome users visiting your site over the past 28 days. That combination is genuinely useful because a synthetic test tells you what’s broken, while the field data tells you what your actual visitors are experiencing.
I find the field data section particularly valuable. If your Largest Contentful Paint (LCP) looks fine in a lab test but is rated Poor in the field, that’s a sign your server is struggling under real traffic or that certain user segments, perhaps those on mobile connections, are having a much worse time than your desktop experience suggests. PageSpeed Insights breaks Core Web Vitals down into LCP, INP (Interaction to Next Paint, which replaced FID), and CLS, giving you a pass or fail verdict on each.
The Opportunities and Diagnostics sections below the scores are where you’ll spend most of your time. Google lists specific, actionable recommendations, such as eliminating render-blocking resources, serving images in next-generation formats, or reducing unused JavaScript. Each suggestion includes an estimated time saving, so you can prioritise the fixes that will move the needle most. At a rating of 4.8 and completely free to use, it’s the tool you should run before anything else.