Yup. I got smoked one day by someone in street clothes on a cruiser with the seat way too low. A couple minutes later, a couple of racer types from the Manly Bulge blew past me. I don't think they caught the guy on the cruiser either.

I can tell, however, that I am most definitely faster on my road bike than my MTB. I think the 80/20 rule may work for this.