[@thewizardguy] I'm sorry but since when were animations in Xcom actually relevant? Normal bullets from the standard weapons weren't displayed nearly as fast as they normally would. And this is coming from a game where you freeze time to make the best approach. I don't think movement and speed are the forte of the series. 

Plus plasma weapons are not only more accurate but have higher precision too. Now I do agree that many plasma weapons in the game are slow enough to even be dodged by normal people, but that's just game animation. It's from the same game where people with basic armor withstand multiple plasma shots in the body. 
 
Finally, this is 'mysterious, super advances alien technology'. With open ended descriptions like that I don't think it's an issue to give super reflexes and normal Muzzle velocity to MASSIVELY superior weaponry. 

Now if you want to argue that they didn't have time to react to it despite the speed of sound being pretty typical as Dreaming said, I can work with that. I don't mind.