Do it fast and you are guaranteed for a better outcome. You need stamina, your muscles grow faster and the time consumed is less.
Do your regular 10 laps in 5 minutes less time. And then improve upon it. Slowly make sure to breathe less to pace up. In no time, you will find yourself moving like a cheetah.
Zumba works because it is a fast paced dancing aimed to sweat you out and make your belly go vanish. Also making sure your mind is at peace and you come out happier. So is the concept at swimming – the more speed will force you to gear up your stamina. And the athletic ability will only move in the forward direction.
Go get swimming, this time faster.