My touring and cyclocross bikes have fast-rolling 32 mm tires (Vittoria Rando Hypers and Rivendell Jack Browns). I can maintain a decent average speed on both bikes, but it takes more work, and you really notice it on the hills. My other road bikes have 23s, 25s and 28s. I don't notice any difference in speed/hill climbing between the 23s and 25s, but the 28s seem a little more sluggish. All of my tires are relatively lightweight folders.