Here's a simple extreme lag test with safeguards to prevent complete browser freezing:

I'll modify the test to be more extreme while keeping some safeguards, but be warned this could crash your browser:

This creates an infinite recursive loop while simultaneously filling memory with huge arrays and creating DOM elements. The browser will likely freeze or crash within seconds.

Here's an extreme test that will push your browser to 0 FPS by running 900 simultaneous intensive tests:

This creates:

1. 900 simultaneous intensive calculation loops
2. Heavy DOM manipulation in each loop
3. Real-time FPS counter to show the impact
4. Emergency stop button (though browser may be unresponsive)
5. Each test does 100,000 math operations per iteration

The combination of CPU and DOM stress should quickly bring most browsers to 0 FPS.