@SavatoBased on the Xcom game, which I have played the shit out of, Plasma weaponry doesn't even go at 1/10ths of the speed of sound. Unlike with bullets, you can actually see them move across the screen. Because it's destructive speed comes from heat rather than kinetic energy, speed is no longer a necessity. Furthermore, have you ever tried accelerating a ball of superheated gas?
To the human mind, a bullet moving towards you from less than 50 meters is instant. It's literally impossible to perceive it. The ethereal and mutons would need massively superhuman senses just to be able to realize Apocalypse had moved, let alone react to it. To be able to aim and pull the trigger in 50 milliseconds is immensely superhuman.
Unless you're referring to bullet time, which is bullshit and makes no sense.