Mark Cuban likes to call Tower Paddle Boards, a San Diego-based company that sells stand-up paddleboards, one of his best “Shark Tank” investments ever.

The company’s growth explains why. When Cuban invested $150,000 for 30% of the company in 2011, Tower Paddle Boards just had $260,000 in revenue. This year it expects to hit nearly $10 million in revenue.

But there’s another reason that makes the 11-person startup special: Tower Paddle Boards is super efficient, having achieved that growth by working only five hours a day.