Its all in the gearing, wheel weight (my opinion) doesnt matter as much as some say, my daily rider is a 26" MTB i converted to drop bars i run a 50t large up front and a road cassette in rear 11-28 i do lose a little top end with the smaller wheel size but its plenty fast very close to my actual road bike. And like said before they may all be in the "same" gear but different size sprockets. Also different crank arm length, riding position, weight, condition of said bikes(bearings and such), so basically there is a lot that makes for "speed gaps"