Knobby, heavy tires are what could really slow you down. I agree with others about looking at a sport tourer like the Salsa Casseroll, Soma ES, Gunnar Sport. All of these frames would easily handle 28 mm tires with fenders. You want some fast, light tires for a commute that long -- something like Continental GP 4 Seasons or Gatorskins, both of which come in 25s and 28s and are very durable and relatively light.