Mouseover highlight lagg

So I went and started a custom game and everything was fine at lowest settings with 60 FPS. Then when i hover over a minion or turrent my 60 FPS goes down to 3. And Back up to 60 FPS after i stop hovering over it. I can attack minions with abilities like tryndemere e withought hovering over them and still have 60. Can someone help me? (CPU usage is low)