I'm about a 50s quicker (for a 10) on my TT bike than on my road bike with tribars.
Combination of the following probably (1) the wheels (spoked) on my TT bike are better quality and I use better tyres (2) I can get that bit lower on my TT bike (3) it's that bit lighter, looks a bit flasher, and makes me feel faster.
Difference between my roadbike with and without tribars averages out at about a minute (not taking into account weather or form).