Sure, the shorter rear end and steeper fork does change the feel of the bike (this one is indeed a lot "twitchier than any of my other road bikes), but that shouldn't have a significant effect on speed. Even going downhill, this bike is slow. ( to put it another way, a LOT more effort is required to attain the same speed as on my other road bikes, even downhill). FWIW, the geometry of the frame (main triangle tube angles and lengths) are virtually identical with my TdF, only the wheelbase is different. Both are d/b steel.

I'm wondering if I'm using tires which are too narrow? I have read in the past that tires which are too narrow can cause a loss of efficiency. Maybe that's the problem??