MAJOR IMAC GAMEPLAY BUG

Just got a new 2017 iMac and I know that this computer obviously isn't a gaming beast. It should be able to handle League of Legends just fine. This bug seems to happen every time I try to play. I get into the game and my fps is perfect. It runs at around 160-200 fps but after a couple min of gameplay the fps drops significantly. This makes the game unplayable for me since it lags so bad when the fps drops. It goes down to about 20 or so fps, which is RIDICULOUS! An iMac with specs that I have should not be experiencing this amount of lag. I've tried every setting combination and nothing seems to be working. Also another thing is that every time I click EXIT GAME it seems to crash the client and my computer. I have no idea why it does this. please comment any suggestions or riot please have some sort of fix for this soon because league is unplayable for me at the moment.