Currently I have extreme lags and stills. This is only since the 9.21 patch.
Please fix it, its so annoying, because I always have to restart the whole PC to join again...
**UPDATE:** I just ran the Hextech Repair Tool from Riot Games and reinstalled the game using the Repair Tool. I recommend you to try it out. I now have stable FPS again.
It is important that you check "Reinstall". But don't select "Clean Uninstall", otherwise it deletes all files from the Riot Games folder after uninstalling the game. It worked for me. Good luck to you!
If you dont have the Hextech Repair Tool, you can download it on the Riot Games Support Site!