League of Legends slows down my computer even after I close the client

So I'm playing League of Legends with my friends and my computer runs it fine at 80 fps. For some reason when I'm playing in-game, my fps randomly drops to around 30, and when team fights break out it goes even lower into the 20s. When the game finally finishes I'm brought back to the client which takes much longer to load the end game stats. It stays this way for every subsequent match I enter. When it's finally done loading, the client becomes unbelievably slow and it takes forever to interact with the buttons. I tried closing the client and reopening it to fix the problem but the client remains slow. On top of that, my computer starts being slow too. Opening google chrome takes three times as long and searching for files takes an eternity in windows explorer. I am able to fix the problem by shutting down my computer but you can imagine how annoyed my friends get when I have to shut down after potentially every game I play. Is there any way to prevent my computer from slowing down in the middle of a league match, or even a way to fix this problem after it occurs?