Customers don’t like waiting, and the country’s largest retailer knows it.

“The No. 1 complaint people have about Wal-Mart is it takes too long to give you my money,” says Charles Fishman, author of “The Wal-Mart Effect: How the World’s Most Powerful Company Really Works.” 

The problem, says Fishman, has a simple fix — more workers at the checkout line. But increasing the hours for workers who’ve just been given raises can create another problem altogether.

“It’s more expensive to pay people more,” he says. “At least at the start.”

Wal-Mart has a plan in place: to spend an additional $1 billion a year on wages. But “we are going to be controlling costs, that’s what we do at Wal-Mart,” says Kory Lundberg, a company spokesman. And that includes trimming worker’s hours at what Lunderberg says is a small number of stores using staff significantly above the hours budgeted to them.

The country’s largest retailer is trying to reinvent itself, says Enrico Moretti, a professor of economics at the University of California, Berkeley, “from a low-margin, low-profit, very cheap store, to one where customer experience is better.”

But that costs money.