from my experience, the biggest ingredient in bicycle speed is the engine. yes, effort for effort, a light-weight aerodynamic road bike will go faster than an upright dutch beast, but a lot of the time in my daily riding, my speed is more a function of how many green lights i catch, how much traffic i have to deal with, etc., as opposed to flat-out, balls to the wall, 110% exertion, maximum power output riding. so on my daily commutes i'm giving the bike a pretty decent amount of power, but it's by no means extreme, so i'm in that zone where the speed differences from one bike to another aren't gonna be as great as they would be to a pro racer (ie. a pro racer could make my road bike go A LOT faster than i can make it go).