Make sure you’re not limiting frame rate in the sim via v-synch in your in-sim settings. It will then split that limited amount of frame rate between your main window and popped out windows. So 60 fps limit will give you 30 for your main sim window, and the other 30 divided amongst the other popped out instruments.
Without V-Synch, I see about an 8 fps hit for the initial pop out, and a cost of 1-2 fps at most for each additional instrument popped out.
Known by users. I don’t thing it’s ever been acknowledged by Asobo.