I assert that one exception to this is compositing. Even if you're on a gaming rig with a really beefy GPU, you might not notice that DE compositing is on, but

I assert that one exception to this is compositing. Even if you're on a gaming rig with a really beefy GPU, you might not notice that DE compositing is on, but you will notice the difference once you turn it off. To test this, try dragging a window around on a high refresh rate monitor on LXDE (since that DE has no composition by default, iirc) and then on GNOME, KDE, et al. The overhead of composition extends to games as well, which is why I like KDE for being the only full-featured DE that lets you turn composition off.