I've used https://isitsnappy.com/ on the iPhone to do these kinds of measurements pretty quickly, to substantiate why I sucked at Steam Deck rhythm games when connected to a TV.
(I do not suck, my setup had 100ms lag)
(I do not suck, my setup had 100ms lag)
You can learn to play Space Channel 5 by tapping 100ms ahead of the beat. But I worried that if I succeeded too much, I'd end up doing everything that way: jumping red lights, taking espresso shots while they were still being poured etc.
...what it's like to experience video game lag in real life.