Okay, so you would obviously like me to carry on and talk more about responsive web design performance testing.
Each developer has their preferred set of applications, or tools, that they use to work with, and testing is no different. There are literally hundreds of tools to choose from. One of the most basic things you have to become familiar with is the waterfall chart. It will be a great asset when you develop and test your web site.
Some other great tools to make use of are online web testing, viewport previewers, and speed-testing tools
Let's start with speed-testing tools. One of my favorite tools is actually made by the search and advertising giant, Google. You can get the add-on from the Chrome App Store or, if you're not using Chrome, you can access the online version at https://developers.google.com/speed/pagespeed/insights/.
Here is a screenshot of a sample result from a website: