You're comparing apples to oranges with this. I have a 3 steel bikes, the geared road bike which is a lugged bike, the track bike and my mtb aren't lugged. Also you can't compare them on a repair stand, you don't get the full effect. Go out and do a 40 mile road ride on a road bike properly setup and you are feeling pretty good, the next day or so go out on the same 40 mile ride on a mtb set up and I am pretty sure that you would be feeling a bit spent from the effort. There's also the rolling resistance of a 26x1.2 tire, the amount of rolling resistance in a 26x1.2 tire is significantly more than what can be had on a 700x23c or in my case with the geared road bike, a 28x22(sew-ups). A 1.2inch wide tire equates to 30mm wide.
Wheel size has a huge factor with it.
Also you have to remember though, that mountain bikes are designed to be efficient offroad, which is a completely different beast than road efficiency. All in total, if you want to be efficient on the road, get a road bike or a flatbar bike, like a trek fx.
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Appreciate the old bikes more than the new.