I have come to suspect that many bicycle commuters create extra problems for themselves because they erroneously focus on speed and light weight.
An article in British Medical Journal attracted a lot of attention when a doctor published his discoveries after keeping good records for his bicycle commute times comparing a very light weight with a medium weight bicycle. Essentially, the author, Jeremy Groves, was surprised to discover that on his relatively long daily commute, the lighter weight bicycle gave no advantage in the real world of daily traffic.
My short experience in Cuba has led me to appreciate the advantages of avoiding an automobile like commuter attitude that overly focusses on attaining speed.
I have recently been following a writer's blog about his experience in San Diego, CA, taking up the challenge of commuting to work every day for a month. Several things this writer focusses on reminds me that he too is naturally (for a person with a US cultural experience) trying to make his commute fast, and in the process perhaps adding extra problems. Luckily, this writer is quickly learning that going fast may be a mistake. He learned that the shortest route may not be the best route. He mentions the safety advantage of riding in a more upright position. But when the author, Robert J. Hawkins, mentioned the need for shower facilities at work, this reminded me of Jeremy Groves' article and led me to use a simple bicycle energy calculating web page to make some quick and easy comparisons.
So, imagine a typical bicycle commute of 6 miles in an urban environment. In that 6 miles there are going to be stop lights, and other traffic factors that limit one's speed no matter how one rides or whether one's bicycle is super light. So, in a best case scenario for this distance, suppose there are 5 miles that one can actually go as fast as one wants. I know my personal limits well enough to pick two energy outputs that cover a range of choices: one at 150 watts, and one at 75 watts. The 150 watt level will get me warm and sweaty and challenge my endurance, but many fit people could sustain this level. The 75 watt level, on the other hand will be somewhat leisurely for a fit rider. In the calculator, I have given the 150 watt rider the option to ride on the drops, and have a 22 pound bicycle, while for the slower choice I have opted for sitting upright and having a 42 pound bicycle (thus giving me lights, spare tube, tools, extra clothes, sturdy tires, a box for carrying things, etc.)
In this very idealistic case, at the end of those 5 miles, the difference in time is about 8 minutes. On the one hand I arrive rested and ready for work, on the other hand I arrive needing a shower, which is probably going to take me more than those 8 minutes.
So, going slow, being safer by having an upright view, choosing a bicycle with less chance of having flats and giving problems along the way, all this may actually save me time and perhaps significantly enhance my ability to commute day after day, season upon season. And best of all, if I just want to get in better shape, I can still ride home hard and fast and sweaty, and the fat tire bicycle still gives me a good workout.
I actually have done this experiment, inadvertently over the years. In my earliest and youngest commuting days I quickly strove to push the limits and sought the lightest fastest bicycle I could afford. As I gained proficiency in fixing flats and repairing rims, I started also opting for fatter tires and eventually opted for my wife's much more practical touring bicycle. Then I found how much nicer it is to stop and get groceries by having a practical way to carry them. I found the need to be ready with lights and rain gear, and in the long run, I learned that those extra seconds I tried to trim from the commute time were not really all that relevant.
So, by all means, lust for that super fast bike, and find the bicycle plan that fits your ambitions, but take a step back and make the effort to think like a bicycle commuter, not like an automobile commuter.