Press a button on your controller to start
Currently works up to 1000Hz with firefox only (as of 30.1.2020).
Gamepad api updates happens only between 4ms in Chrome and other Browsers, which limits the updates to 250Hz in them.
The values are rough approximations, but will give you clear indication if you're around 250hz or 1000hz.
Move both thumbpads in circular motion as fast as you can to find the peak values.