I have been passed by another commuter only once. But that wasn't due to superior speed, but somewhat poor judgement, on the other person's part.

On a downhill section of my commute home, I stopped for a red light, where seconds before, a young woman on a typical commuter bike had stopped next to me. As soon as the light turned green, I sprinted away, like I always do. Still going downhill, I had to slow down to make a turn, then soon after make another turn, and then had to slow to let crossing traffic pass. That was enough for the other commuter to almost catch up. I darted off again and approached a red light, so I slowed down, but it turned green before I reached it so I kept going. She was still close behind (maybe 100-150 ft back).

Onto a much slower street, with virtually no room to pass cars and frequent speed bumps. 10 seconds later, she was right behind me and going the same speed (which was very low at this point - 10 mph maybe). The car we were both behind had to brake to let pedestrians pass, so I stopped, because there was almost no room on either side of the car to pass. Then she slid past me on my right and squeezed herself in beside the car and the edge of the street (about 8" high sides). She couldn't pedal, so she "walked" the bike past the car (almost scratching it), and pulled away off ahead.

At first I was amazed that she kept up with me so well, but when I thought about a bit more, I realised how much time slowing down for traffic and stopping for red lights really eats up. Yet, still she "won"!