League of Legends FPS Issue

So I've been having an FPS issue since pretty much the entirety of the preseason. I used to run the game on smooth 60 fps and now run on around 30-40 for no reason at all. Or at least that is what I thought. I noticed today that my league has actually been running in borderless even though I have fullscreen enabled. If anybody has had an issue like this and would know a fix I would really appreciate it. I would also like to add that yes, I have tried switching between fullscreen, borderless, and windowed, and they all run in the same frames and function the same in terms of being able to alt + tab.