I started with the 32mm Conti Cross race tires that came with the bike and put on a 22-24mm set of slicks for a century and noticed a huge improvement.

Then I went from those to a set of 42mm Cross Rides and noticed almost no difference in rolling resistance or speed, only a higher sensitivity to crosswinds. The Crossrace has an interrupted center tread pattern, the Crossride a continuous one.

It's all about the tire design and inflation pressure. Width has little to do with your speed potential.