HTTP Debugger is a professional HTTP sniffer and analyzer for developers.
You can use HTTP Debugger to debug HTTP API calls to a back-end and between back-ends.
HTTP Debugger is very easy of use, with clean UI, and short ramp-up time.
It's not a proxy, and does not produce network issues!
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.
Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
PageSpeed Insights - PageSpeed is addon for ...
Surge for Mac - Advanced Web Debugging Proxy for Mac & iOS
WebPagetest - Run a free website speed test from multiple locations around the globe using real browsers...
HTTP Toolkit - Beautiful, cross-platform & open-source tools to debug, test & build with HTTP(S). One-click setup for browsers, servers, Android, CLI tools, scripts and more.