WebPageTest is the tool performance engineers reach for when they need to go beyond a headline score and understand exactly what is happening during a page load. It was created by Patrick Meenan (now at Google) and has been the gold standard for serious performance analysis for over a decade. The interface isn’t as polished as GTmetrix but the depth of information it provides is unmatched by any other free tool in this category.
The test configuration options alone set it apart. You can choose from dozens of real test locations around the world, select specific browsers, define your connection speed (including custom profiles), and configure advanced options like custom JavaScript to inject or HTTP headers to send with the request. The first-view versus repeat-view comparison is particularly useful because it shows you the impact of caching: a page that loads in 3 seconds on first visit but 0.8 seconds on repeat visit tells a very different story than one that’s consistently slow both times.
The filmstrip and video comparison features are outstanding for visual performance work. You can run tests against multiple URLs simultaneously and generate a side-by-side filmstrip showing exactly when each page becomes visually complete. This is brilliant for comparing your site against a competitor or measuring the before-and-after impact of a specific optimisation. The Waterfall view is probably the most detailed available in any tool, with timing breakdowns for DNS lookup, TCP connection, SSL negotiation, Time to First Byte, and content download for every single resource. If you’re serious about web performance, you need to know how to use WebPageTest.