That slowness is very common, AFAIK. I have it, too, specifically if I try to punch things. Good thing, too - a couple of times that has failed to work and I've woken up just in time to see my first punch a wall or my elbow smash into the radiator next to my bed. Hurts.

I don't know if it's related, but sometimes in my dreams I fly by taking advantage of this oddly sluggish momentum. All it takes is to launch myself into a spinning motion like a frisbee, and I'll fly a good distance.