15 min is a significant time-saving!! How far is your actual commute? On the rare occasion I get a tail-wind into work it saves me no more than a few minutes off an 18m route!
I am currently going by MTB, rather than my road bike.
Thursday took me 1 hr 10 for a 15 mile Journey.
Friday morning, 55 mins
My all time record, on the road Bike is 45 mins, but I think that is under threat this summer because what I have noticed is that the MTB is so much harder than the road bike, and this is the first month of 30 that I have ever gone by MTB.
My legs are so much stronger already, and my normal lazy left leg (on the road bike) has become the more dominant on the MTB for some reason !