Dretches aren't fast at all, it's the combination of a high Field of Vision and being close to the ground that gives the illusion of speed. The Dretch is actually among the slower of the Alien classes.
The dretch base running speed is the highest of any alien, so your statement is only true where circumstances allow the larger classes to exploit their movement abilities like pouncing/charging. A marauder with no walls to jump off can easily be outrun by a dretch.
Naturally, I talk only about real pratical situations, and not a fairytale land where Dragoons do not pounce, Marauders do not walljump and Tyrants to not trample. Since the only things a Dretch can do a jump and wallwalk, they are indeed among the slower classes.