I started commuting 15 km each weekday about six months ago, and I've noticed that I have slowed down. I think it's for several reasons: more than before I don't like breathing in auto exhaust, and thus seek to avoid breathing as heavily, riding fast causes a bit more mental stress, I don't want to hit the bumps as hard, I'm a bit less wreckless riding in traffic, and during winter I don't want to go as fast in order to reduce wind chill.
I say as long as you're satisfied with your health, don't worry about your speed.