Tap any key two or more times. The milliseconds between the key down action will be displayed below. More precisely, any combination of keys that don't activate something else in a browser. Control works fine, for example.
Note to apprentices: something I've whipped up in the last few minutes (2020/12/20 5:30pm EST / GMT -5). Soon to become a reverse metronome. This is all client-side JavaScript, so you should be able to download it, run it in a browser, modify it, etc.
I'm going to link to a specific "commit" / version because I'll likely move the code eventually. You'll have to navigate around the verisons.