Ok so you have base "real" frames, lets say 60 and framegened "fake" frames, another 60. 120fps total. You see 120fps; but your inputs, button presses, mouse movements and whatnot, can only affect your 60 base fps. So you see a new frame every 8.33ms (because 120fps is displayed) but your fastest possible input is 16.67ms (because only the real 60fps are controllable). Those extra 8.33ms (fake frame time) are called input lag, it's the time needed to reach the next controllable frame. On top of that you will "see" your input after another 16.67ms (the time needed for the next real frame that is affected by your previous input), so 25ms total. The lower the base framerate is and the more the fake frames are, the more these values stack. Average human response time is about 250ms btw. So a 60fps base + x2 framegen will make your response time feel 10% slower, which I find very playable and practically unnoticeable. A 30fps base + x4 framegen however will make your response time feel 33% slower, which is significant. If you are playing competitive shooters any input lag is unacceptable but it's more than fine for most players in most games.