While I was ingame yesterday, the FPS suddenly dropped to around 30 and stayed there for the next 2 games. I decided to reinstall League today in hopes that it would fix the issue, but nothing changed. I updated my graphics card drivers (Nvidia GeForce 750ti), but that only resulted in a 10 fps boost, leaving me stuck at 35-50. While it at least seems playable now, the difference is still noticeable. I even changed all of my video settings ingame to the lowest possible, but that didn't affect the FPS at all. I've also tried to repair the game through the client, but that didn't change anything either.
Edit: I've tried some games that are more demanding than league (Warframe and Elder Scrolls Online) and I got 60+ FPS in both, so I'm assuming it's a league issue.