My primary commuter is the child of a cyclocross bike and a touring bike. In the three-seasons I run 28s on it. It's equally as fast as my roadies, just not as quick. In stop-and-go city traffic, there's a bit of a difference, maybe 10%, which amounts to a minute or two on an 18-20 minute commute. When I can take my long loop (16.5 miles each way) which has far fewer stoplights, there's almost no difference at all.