There are some features that other site speed tools don’t have. At least for free.
“WebPagetest is a tool that was originally developed by AOL for use internally and was open-sourced in 2008 under a BSD license. The platform is under active development on GitHub and is also packaged up periodically and available for download if you would like to run your own instance.
The online version at www.webpagetest.org is run for the benefit of the performance community with several companies and individuals providing the testing infrastructure around the globe.”