I've gone the rounds from an MTB on knobbies to slicks, to a road bike, to my current commuter my Bacchetta SWB Recumbent (Corsa). Each time saw a drop in my commute time, and a drop in effort.
With the winter coming up, which will mean the occasional rain here in San Diego, I'll probably use my LHT as it's fendered, and give myself some extra time. The MTB netted an average speed of around 13-14, the road bike would have me at about 16 give or take, and the Bent has me doing the commute at 18-19 average. The nice thing I like about the Bacchetta is that I have a "Brainbox" that I can throw my junk in, and hydration is taken care of by a fastback hydration system. No panniers involved, and I have enough storage for my commute. If people ask me about a commuter I generally point out the Giro which is a powder coated dual 26", and can take the abuse of commuting, plus the price is a bit less than the Corsa. I went with the Corsa as I do weekend rides and distance stuff, and wanted the little bit of extra speed it offered. Easy 3-4mph increase in speed for the same amount of effort. Oh, and it can climb, but that's more to do with the engine. I was definately slower up the hills when I first started, but now I'm pretty close to my upright speeds.