Burger King is outperforming McDonald's and Wendy's. 

The fast food chain is posting US sales gains at a time when its competitors are falling or stagnant. 

Burger King owes its success to one thing: simplicity, according to a recent report by Citi Research. 

"Burger King has debuted successful new limited time offers without adding back-of-the-house complexity," Citi analysts write. "The brand has highlighted simplicity effectively — adding sauces, cheese, or when in doubt, bacon— and continues to benefit from successful promotions (2 for $5 sandwiches)."

By creating new menu items from existing ingredients, like the new ultimate bacon cheeseburger, Burger King can entice customers without overwhelming its workers and slowing down service.

For instance, the ultimate bacon cheeseburger is similar to the classic. It contains burger patties, bacon, and American cheese, and is topped with A1 sauce. 

"Nearly all of these items were already present in other menu items but the unique combination creates a marketable new item," Citi writes. 

McDonald's drive-thru wait times have gotten longer, a problem that experts blame on an increasingly complex menu. 

Burger King is also promoting the items already on the menu. 

Cash-strapped Americans are increasingly unwilling to shell out for fast food. 

Burger King has lured in customers with promotions like the two for $5 sandwich deal, Businessweek writes. The brand also sold 10 chicken nuggets for $1.49. 

To help offset the discounts, Burger King raised the prices of some items. 

While McDonald's offered a promotion on free coffee, the higher price of burgers is a turnoff for consumers. 

A Double Quarter Pounder with cheese, fries, and a drink now totals about $7.50 at some Chicago locations, Bloomberg reports.

That's too expensive for many customers, Leslie Patton at Bloomberg writes.

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