Performance is not just a simple number. 25 FPS with good frame pacing is much more enjoyable than something that averages 60 FPS but with individual frame times all over the place. That said, for first-person action games especially on a non-tiny monitor, anything below ~40 FPS will be noticeably non-smooth. Other game types have more tolerance, e.g. a top down strategy game could still be playable at ~15 FPS.