fuji, do you have a picture of your bmx? I was expecting it to be slower than my road bike, but not super slow with this set up. The gallery I linked to above is a guy using his as a courier.. so that has to be worth something.

I was thinking slick tires, pumped up a little to high, and as aero of a seat set up as possible given the short seat-to-handlebar distance (I was thinking a layback seatpost to help with that one)

I know the ride would be really harsh on 20" wheels... factoring that in.