Practice counting down a deck

I have read many accounts of people saying that they practice their counting skilld by counting a single deck one card at a time, with the objective to end at zero when done. They say that the really good counters can do this in about 15 seconds.

I have tried to flip through a deck in 15 seconds, without counting, just flipping the cards as fast as I can, and my best time is about 20-25 seconds. I am starting with the deck in my left hand and taking one card at a time off it with my right and putting that card down on the table. Then repeat 52 times.

Am I doing something wrong? Or are some people just incredibly fast at dealing cards?

Thanks!