That explains everything. With anything but the very slightest downhill the energy assist from gravity far outweighs mechanical losses in our bikes. It's rider effort plus gravity against mechanical loss and the wind. If the hill is steep and long enough gravity alone will be enough for us to reach 30 MPH.

Riding into the wind on the flat our lower speed reduces mechanical losses but not by nearly enough to compensate from losing that wonderful force of gravity. It's just us against the wind unless someone will provide a draft.