I would assume it’s because a flat screen only renders one window while a vr headset renders two at a wider FOV. On the quest 2 it’s 1920 X 1832 per eye, so a total of 3840 X 3664 thats higher than 4k at 3840 X 2160. Also because the two displays are offset you are rendering two different pictures for each eye instead of one which will have it’s own geometry associated with each eye vs a single geometry render for a flat screen.