Send it to your site and it'll tell you about what's making your site slow and how to fix it, and the list is in priority order so you know where to start.
Great for themes (e.g. picks up if you've forgotten to fully compress your images), but also good for testing your Moodle installation (e.g. it'll check if you've got gzip turned on).
PS I just created this exact post, word for word in the Themes forum, so if you want to talk about using this tool to make your Theme faster, you might want to go there instead:
We've been using this tool for a while to speed up some of our custom code. This tool does indicate several things that the Moodle developers should do to increase performance:
The following image(s) are missing width and/or height attributes.
http://www.myaddress.com/moodle/theme/image.php/standard/core/1375416055/moodlelogo (Dimensions: 100 x 30)
The following publicly cacheable, compressible resources should have a "Vary: Accept-Encoding" header:
Optimizing the following images could reduce their size by 503B (25% reduction).
Losslessly compressing http://www.myaddress.com/moodle/pluginfile.php/672/user/icon/standard/f2?rev=1 could save 503B (25% reduction). (These are the default user image files in Moodle. They aren't optimized).
- Optimize CSS delivery for the following resources:
Some of the things you can do to make the theme CSS smaller, and therefore faster is to for example, manually delete RTL rules if you're not using them, delete Moodle modules you're not using (since Moodle will stick the CSS in anyway), run the CSS through a reformatter to make it more compressible. There are various bugs filed about these issues too, mostly linked from MDL-39094
If you're handy then some of them can be done manually to trim down the CSS size.