Severe FPS lag

I get severe FPS lag in-game, sometimes as low as 4 FPS. This has been a problem since the beginning of S8 but wasn't an issue in S7.

I'm on a 2009 Apple iMac, running OSX 10.12.6. I've lowered video settings and everything that has been suggested. But the problem still persists. The more there is on screen, the worse it becomes. ARAM is atrocious to play. SR gets severe FPS lag during team fights that I can't see anything.

Any help is appreciated.

Also, the Hextech Repair Tool no longer works. It doesn't finish gathering logs no matter how long it runs. It wasn't an issue two weeks ago.