I'm wondering whether this live blur effect might have played a big part here. Since its full screen, macOS doesnt need to render all these fancy effects. I'd like to see similar tests with live blur turned off.
Actually, I remember reading something that Windows doesn't do any blitting / buffer swapping if you just leave it idle and no updates happen on screen. However, there are many ways to throw this off and force drawing every frame if you are not careful.