This is marked as minor just because it only happens after a few hours. But when it happens fps drop to 5-10 and never recover.. So when it happens it's a major issue.
It's not easilly reproducible and profiling build report doesn't hold much more info than sound->ssAdv being very heavy. I've identified the function that has the highest execution time while that happens. See Additional info.
FPS immediatly recovers when you tab out. Because sounds stops being processed then.. So I think this is also known as the Alt-Tab bug.
This bug has only recently appeared. I can't remember it happen <1.66