Yes, but the same is true in Wayland and it's automatic as well. Full screen applications don't require compositing so they typically don't, and this was the case in MOST X11 compositors as well.
Turning off compositing before launching a game is maybe a bit misguided, sort of like turning on the "classic theme" in Windows 7 before launching a game. It might save some memory, sure, but latency should be identical.
Those results depend on having a 500hz monitor. There's a world of difference between a mildly-buggy rendering loop's 1-frame delay or missing a vsync deadline causing a 2ms delay vs a 16ms delay.
Turning off compositing before launching a game is maybe a bit misguided, sort of like turning on the "classic theme" in Windows 7 before launching a game. It might save some memory, sure, but latency should be identical.