Could be a combination of the tires you're running and other things such as traffic lights. Since you said you're slower than other roadies, you might look into switching tires to something that rolls faster such as Continental Ultra Gatorskins in 25c. Not going to be as comfortable, but I know your speed will improve. My wife run those tires on her bike and was constantly complaining about the bike being sluggish. I talked her into switching to the Gatorskins and she was very impressed with how fast the bike was after the switch.

I run a 2006 Tricross Comp as my commuter and have 25's. My 22 mile, one way commute takes me about 1.15 hours. I don't have that many lights though and it's pretty flat. YMMV.