Bike wheel sizes are weird. I did not realize till recently, but the wheel size refers to what the outside diameter (OD) "should" be with "normal" tires. it's a ridiculous tradition refering to the nominal size of the tire, not the rim, unlike car or motorcycle or anything else I'm aware of.

mtb: 22" rims
road: 622mm (24.5") rims

so anyway: say a speed-oriented commuter used 25c tires on his 700c wheels, the diameter of his wheel would be 26.5in. If the same guy used 1" tires on his mtb, the wheel diameter would be 24 inch. using equivalent rims etc, the smaller wheel would have around 20% less inertia. this is just a guesstimate. the road bike not only has the heavy parts of the wheel farther away from the axis, but since the rim & tire are also physically larger, everything else being equal, they weigh more too!

anyway, this is mostly conjecture. road bikes are obviously more fit for the road than any mtb, and if smaller wheels made road bikes faster, they probably would have adopted them. Smaller wheels have higher rolling resistance also, which comes becomes a bigger factor the longer the rides are.

as it is, wth rough roads & a lot of starts & stops, the mtb feels more suited for my situation. if i ever get smoother roads & fewer stops, you can bet I'll be on a road (or cross) bike.