Having FPS drops right when i installed Yosemite.

Hello, right when I have installed Yosemite on my imac. I've been getting 30-40 FPS ( Constantly changing ). It was fine when I had Mavericks.. I don't know what to do, I've even tried lowering my setting to the lowest and still gotten 30-40 FPS. My usual FPS in league was 60 and it was fine, it never dropped below 60.