Comment by VRay

you can just point a phone camera at the keyboard and screen, record a video clip, blow it up into stills with ffmpeg, and then count frames

Even at 60fps, you should be able to see the difference between 30ms and 100ms. Newer phone cameras often have a "slow motion" mode that'll give you 120+ fps video, too

SOME people can see the difference between 30ms and 100ms, but it's almost impossible to actually see it. You'll mostly end up feeling that everything sucks and not knowing how or why.