When Five Guys Burgers and Fries stores started opening in San Diego, Calif., this spring, lines spread out the door for days on end as burger-hungry customers who like fries cooked in peanut oil welcomed the franchise chain to the community with hearty appetites and demand.

Customers order at Five Guys stores just like they do at traditional fast food restaurants, telling cashiers what they want and picking it up on a tray when it's ready. But it's not traditional fast food restaurants like McDonald's and Burger King that Five Guys is hurting the most with the success like it's had in San Diego.

Full-service, mid-priced restaurants including Chili's, owned by Brinker International Inc and Applebee's, owned by DineEquity Inc., are struggling to keep up with the shift in customer tastes and desires that's leaving sales flat while the hungry flock to Five Guys and similar so-called "better burger" restaurants and other competitors in the space, including Chipotle Mexican Grill Inc. and Panera Bread Co.

These restaurants serve like traditional fast food restaurants, but provide foods made with better ingredients. The waits are short and diners don't have to pay tips to servers, but still get to enjoy higher-end food.

"Chili's and Applebee's have such a negative connotation with just being cheap food," said Sarah Perry, a 27-year-old arborist at the Morton Arboretum in the Chicago suburb of Lisle, Ill., in an interview with Bloomberg. She said she would rather "dash into a Five Guys" for a burger rather than sit down and order at a casual dining restaurant.