Using WebPageTest

Chapter 4. Comparing Tests

By now, you should have a good idea of what to look for when optimizing a page. Supposing you have compressed a few images or delay-loaded a JavaScript file or two, you’re probably eager to find out how much of a difference these changes have made. Besides manually loading each individual test’s results page side by side, WebPageTest allows you to compare tests with tools especially designed to highlight the differences. In this chapter, we’ll look at the comparison tools at your disposal and how WebPageTest makes it all possible.

Perceived Performance

So far we’ve looked at web performance from a very mechanical point of view. That is to say that we’ve gathered a lot of great data about how the page is constructed “under the hood,” like millisecond-precision event timings. But there’s one area of web performance that is arguably more important than simple load-time metrics. That is the measure of how fast a user perceives the page to load: does it feel fast? So what’s the difference? For example, the user doesn’t care if the page-load event fired after one second if the video she was intending to watch doesn’t load until five seconds. The literal load time of the page was only one second, but the perceived load time was much longer. Overhyping metrics like the load event can lead us astray when optimizing web pages. We also need to look at performance from the users’ perspective and optimize for their experience, which may not necessarily align with ...