When a quick or full scan is launched, use an idle priority
Currently when you do a full scan of a system, it pointlessly eats priority resources until done.
Sure it can make sense that the vital system part is scanned quickly with priority "Normal", But all the user content could just as well be scanned with priority "Idle" or at the least "Background". There is no point in slowing the whole system down for this. If people want it to scan quickly they can just leave it idle.
At the least add a setting, to change the silly default.
The worst problem with current default is that it will defer people from starting a scan frequently (or tempt them to cancel it) as it will interfere severely with what they are doing.