You can put slicks on a mountain bike and pick up a couple mph without too much trouble, if you change your mind. But it will still not be like a road bike, or even a skinny-tired hybrid on the road.

Another thing is that mountain bikes don't have a teeth-jarring ride over rough pavement. They're just smooth. The big, fat tires absorb just about any road imperfections with ease. At the cost of speed.