GTmetrix - GTmetrix is a free tool that analyzes your page's speed performance. Using PageSpeed and YSlow, GTmetrix generates scores for your pages and offers actionable recommendations on how to fix them.
HTTP Debugger - Debug HTTP API calls to a back-end and between back-ends. Easy of use, clean UI, and short ramp-up time. Not a proxy, no network issues!
PageSpeed Insights - PageSpeed is addon for ...
Google Chrome Developer Tools - Get started with Google Chrome's built-in web developer tools.
WebPagetest - Run a free website speed test from multiple locations around the globe using real browsers...
Firefox Developer Tools - Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile.