I always play with MBOIT disabled, it seems the performance will get worse the longer the match goes on for due to a memory leak.
Someone else said reinstalling the game fixed it for them, but I had no such luck.
I know for a fact that the game is using my discrete GPU, so unfortunately that solution would not help me and I'm certain we're experiencing different problems.
I've been experiencing the same issue, reinstalling the game did not fix it. Performance gets worse and worse as the match goes on.
This issue has plagued me since Mirage's release, however. I don't think it's related to the patch that most recently came out.