That sounds about right. I use 26x1.0 front (heck I have a front shock) and 1.5" conti slick in the rear (for better gearing, and flat protection).

I used to ride with road bikes on my mountain bike - from 400 mile tours to 55 mile race training. With the right tires, it is possible. One of the biggest weaknesses of MTB tires is the weight - they can't accelerate from a stop, or climb a hill as well, and all of those make a difference in your average time as well as your mean cruising speed.

Yes, knobbies take a lot of energy away from you, weight is killer in wheels, and Aerodynamics (2.0" vs 1.0" tire) is pretty important if you are getting anywhere near 20+mph. Below 15mph, it isn't much of an issue. It all adds up if you are riding strong. If you are doing 10-15mph on a hybrid or upright bike, probably not so much...