road bike vs mtn bike

First my logs are up to date.

I try to bike commute the 10.5 mi to work as often as possible. I usually ride my 1983 cannondale roadie with clip on aero bars. It usually takes me between 29:30-30:30 minutes to get to work depending on the wind. I have an XTERRA coming up so i wanted to get some MTB saddle time, so today i rode the mtb with its knobbies. My mtb is nothing special, its a full rigid giant atx 830. Both bikes have spd pedals. my ride in today was 31:43 with a cross wind. so my main question is, why am i so inefficient on my roadie? I guess maybe i need to join the list of "how's my fit" posters. and no I have not gotten a professional fit, 2 reasons #1 no one near by that does them and #2 i'd feel a little silly paying $300 for a fit on a $150 bike.