I've been playing for quite a while now, and only recently seen this issue, but today I went into a ranked game, started with ~180 fps (I have a gtx1050ti and I5 7500k), then about at the 18 minutes mark my fps reduced to 60 for no reason. And shortly after it went to 17. I checked my device manager and disabled my other gpu just incase, nothing happened, then I checked the NVIDEA control panel and it said my gtx1050ti was running the game, so I reduced graphics quality from very high to normal and saw no change. But after a few minutes my fps went to about 50. My conclusion is that is has to be a bug within the game. Also my ping was very %%%%%y but that could be due to my unreliable isp.