The game has been fine this entire year and even through the Anniversary Event. Something happened recently that has caused a serious performance drop. The game stutters and has FPS drops a few times a match at least and sometimes is just downright choppy during huge team fights.
This game had issues like this in the past during the first 1.5 years of release but they always seemed to be fixed. I hold out hope the next update magically fixes this again but the game is awful right now.