Sprinter "Fast" Fred Rodriguez used to use original Eggbeater MTB pedals for road racing (3x US champion, 2nd Milan-Sanremo), so there can't be much difference in speed.
There are road-specific versions of the pedal now, but they aren't very different from the some of the MTB versions.
There are considerable benefits in having the same pedal/shoe system on all your bikes so it's any bike/any shoe. Soggy shoes from the day before? - just use another pair. Well-ventilated road shoes too cold? - use your winter MTB boots on the road bike. Pedal broke? - borrow one off another bike.
I use Time ATAC on all my bikes.