Well, for anything actually putting pixels on the screen under the compositor's purview, the compositor controls the framerate of the game and there needs to be context switching involving the scheduler to get things displayed.

I'm not clear on what mechanisms exist today for preventing things like a DoS of GPU resources in something of a single entry to the GPU driver without returning to userspace. We've all seen things like GPU stalls reported in dmesg, so clearly the drivers are tasked with preventing that sort of hang where a process enters the driver and stays there too long.