The furthest I typically travel for transportation is about 10 miles from home, and over time I've noticed something kind of funny: It takes me around 45 minutes to get there pretty much every time, no matter what bike I am riding. I have done the ride on several upright bikes, road and touring bikes, mixtes, single speeds - and it's always the same. If I happen to be a little slower or faster on any given day, it seems to depend more on traffic patterns than on the bike I am riding.
This is not to say that some bikes are not faster than others; clearly there are enormous differences. But when riding for transportation through densely populated areas, I find that more often than not these differences simply do not matter. Because I follow traffic laws and stop for red lights and stop signs, being on a fast bike just means that I am riding faster between those enforced stops. Maybe once in a while I'll make a green light that a slower cyclist would not, but somehow it averages out and in the end I don't really "win" any time.
Of course there are other benefits to being on a faster bike. Hills are easier. Accelerating is easier when going around obstacles or starting from a stop. All things considered, I prefer to commute on a bike that is fast - as long as it's also upright, comfortable, and fully equipped for transportation. But the faster bike does not deliver me to my destination any sooner; the 45 minute rule always applies.