What frameworks and tools do you use to identify and fix a performance bug?

"Even the best-designed websites have performance issues from time to time. Identifying the root cause of these and resolving them is critical to my work as a web developer. When I notice that the website's performance is degrading, I first identify the underperforming elements. I then inspect them visually to see if I can spot any obvious errors in the code. Once this step is complete, I use standard debugging tools to identify errors that I cannot see. I typically use some of the common ones, including Angular.JS, Sass, Grunt, Sentry, GDB, and NPM."