I
got to give you MTB riders big props!!
Off the top of my head, I would say MTB riders have to work at least 40% if not more harder than road bike riders. MTB tires, and weight adds more to the resistance. However, I have been passed by MTB on the flats
If you put dual purpose or slick tires on an MTB you'd be surprised just how much faster and easier it gets. It does take some off the offroad potential away, but for commuting its an ideal modification. I got 28km/h average on yesterdays 150km ride. 95% of that is of course on road.