I disagree about big 2+'' MTB tires not affecting arrival times/speed. Since I switched from Kenda 2.35'' to 1.5'' not only do I feel faster, I am faster. I gained an average speed increase of 1.2 mph by moving to Schwalbe Greedguards. Bigger tires require more energy to get them going. Yes that is a hardly significant number, however the bigger issue is the perceived speed boost, and even more importantly the smaller tires require less effort to keep the bike going I think. With bigger tires I feel the drag (high rolling resistance?) and so don't feel much motivation to push the bike any harder than required.

That said, my MTB works beautifully as a commuter. I'm using a rear rack + panniers with no heel strike.