There's been two really interesting videos released recently that use the "Local Overrides" feature of Chrome DevTools in order to play with web performance without even touching the original source code.
- Umar Hansa: Improving the performance of Soylent.com with local overrides and font-display
- Harry Roberts: Using Chrome’s ‘Local Overrides’ to Test Performance Hypotheses
The big idea is that you can literally edit CSS and reload the page and your changes stick. Meaning you can use the other performance testing tools inside DevTools to see if your changes had the effect you wanted them to have. Great for showing a client a change without them having to set up a whole dev environment for you.