Well, yes. But what is it about a road bike that you think it makes it slower under these circumstances..?

Matt095 is right; it's air resistance caused by his body acting as a sail. To reduce it he'd need either drops, a long stem and narrower bars, or an aero bar. Aero bars will reduce drag most.

Re the suggestion about 25mm tyres: on a smooth surface tyre drag is either rolling resistance, which all things being equal gets worse for narrower tyres, or air resistance. Tyre air resistance is marginal and not worth worrying about except for racing. Especially given the other disadvantages of narrow tyres.