Amumu drops my FPS

Whenever I play JG, the in-game FPS will always start to drop significantly (from 80 to less than 10) during the first 4 minutes or so. FPS remains around 20 after that. What's worse, every game after an Amumu game, no matter what champion i play, the same issue comes up again and it doesn't get solved until I restart my computer. I am using a Macbook Pro.

How to reproduce the bug:

1. Open a league game in Mac;
2. Play in game.

That's it. I have experienced this issue at least 3 times on two different accounts on two different days. Please solve this. Thanks.