I don’t spend 15, but I do spend more than 5. In my case most of the time spent is lathering up, i.e. the time when the water isn’t running. Do you count that in your five minutes? And how do you shave in the shower? Do you have a mirror in there? How doesn’t it get all fogged up?
Edit: Now that I think of it, a small mirror in a shower could work, you simply have to pass it through cold water when you want to shave.