WebPageTest runs your website through a real browser on real devices from locations around the world and produces a detailed report on what loaded, when, and how long each step took. It captures filmstrips, waterfall charts, and Core Web Vitals. You can test on specific browsers, connection speeds, and device types. Tests can be scripted for multi-step flows like login or checkout.

Developers use it to find render-blocking resources, slow server responses, and layout shifts. The results go deeper than most speed-test tools, which makes it worth the slightly steeper learning curve. There is a free public instance and a paid version for private testing. If you care about why a page is slow, not just that it is slow, this is the tool to use.