I have 80 FPS but 60% of the times it feel like it’s 15fps.

Hello summoners, so today I wanted to ask about a problem that I have been having for weeks, and the odd thing is that it was happening on 2 different computers, a Mac, and a gaming computer running GTX 1050 ti, 8gb ram, so the problem is that many times I feel like I’m playing 10-15 FPS, while it shown on the monitor that I’ve got 100-150fps, I reinstalled lol by deleting it completely, changed the setting of lol, changed the setting of nividia, initiated full repair, and looked desperately on forums and I indeed did found very similar threats, but yet no solution! Other games work perfectly fine and smooth, expect for lol! Please help! It makes my games unplayable!