Originally Posted by Revracer

I agree with the OP and Kopsis. I have a Felt F65X CX that was recommended in size 57 (22lbs) for me and a Cannondale Synapse in size 61 (19lbs) and I feel a noticeable difference in that I can achieve 1-2mph more on the Synapse road bike. I am somewhat doubtful of the aero advantage due to the high seat position I ride on the CX bike due to the 57. I have often wondered, is it:

1) 3lbs of difference (I can gain/lose 3lbs and I ride heavier steel road bikes that are still faster)
2) Wheel weight, Felt probably carries an extra pound in the wheels
3) Aero, while 57 makes me get lower, the CX has shallow drops
4) Geometry, there is something about pushing on the pedals of the road bike that feels easier
5) Tire size - I have never tried a 700x23 on the CX, but even rolling down a hill the Synapse is faster
6) Aluminum vs Carbon, is the stiffness of the CX bike burdensome? It should be faster due to stiffness I think

Is has been an interesting consideration for me, as I thought after getting the CX, it could be a be all bike for me, but the road bike (and even a few vintage steel road bikes) are able to maintain better speed on the roads.

Great topic of debate!