you know, I have a great worksman bicycle at my office. It is a huge thing, it weighs 40 pounds. After zipping around town on my little cross or roadbike, it is a humbling experience to hop on to this rather newish bike and ride just as fast as i can (on the flats here at work) and MAYBE get up to 10 miles an hour. What's more, I'd like to see anyone else do 15 mph on the flats on a bike like this.
having said this, I must add that I can do a lot better on a lightweight bike.
So if you are the owner of a comfort cruiser, Do not expect to go really fast no matter how fit you are, because the sheer weight of the bike is going to slow you down.
Your speed on a bike has a lot to do with the bike itself.
Take a look at the kinds of bikes they are riding in the Tour de France.
Those bikes weigh about 1/2 of what most of our road bikes weigh; and they are even built differently, different geometry and stuff.
One of the reason that they can sell those really expensive bikes is because they do add to your speed.
So worry about YOUR speed, not someone else's. but even that is subject to so many variables. Try to beat your own records, but if the wind is going in a different direction, even if you worked twice as hard as you did the last time, you still might not be any faster.

The biggest bummer about speed is that it limits who you can ride with. If you have a friend that averages 15 mph and you only average 12 mph, it won't be any fun riding with them; either they will be falling asleep waiting for you to catch up or you will be working TOO hard to have fun... or both.

if you are riding to get fit, don't compare yourself to anyone else. Just imagine me on my worksman bike trying to go fast.