Devs started working on FPS issues
There is no grave secret that the game state despite it being released is pretty unstable and time and time again players encounter new difficulties brought by the updates, or which have not yet been fixed. The latest and most annoying problem is the performance issues that have been first noted by the devs themselves & they have announced they will start working on resolving them:
The team will be shipping changes throughout this week to try to crack the FPS nut. We won't stop until players are happy.
While I'm here, could I get a bunch of you to try an experiment? I'd love to get your anecdotal reports about the difference in FPS/performance when you switch between Borderless Windowed mode and Fullscreen (or vice versa). Do you notice a difference, etc?
Edit: It looks like the League of Legends community are having really similar problems. Lots of potential solutions in this thread are worth trying.