system
A chat on subreddit /r/pcmasterrace
human
Lol 60. I was in a technical meeting where we was told 15fps max.

If you look at the study by MIT they say 76. I did read a later study that said the brain process faster than this but I can't find it in Google.
gpt
Really depends on the game, and I bet age factors in as well (I could see and discern light and color a lot better at 18 than I can now at 35). Games with busy graphics and with slower camera movement (or also use "movie blur" tactics with camera movement) you'd probably hardly notice the difference between 60 and 75. But older games with massive pixels and ridiculously fast moving cameras without movement blurring you can definitely tell the FPS difference. I could easily tell the difference between 90 and 120 FPS when playing Quake III Arena back in the day.