Fast food just got a little less speedy due to more complex menus, according to a drive-thru performance study released on Monday.
The average speed at six benchmark chains and one regional chain (Burger King, Chick-fil-A, Krystal, McDonald's, Yum Brands' Taco Bell, Taco John's and Wendy's) rose to just more than three minutes, which amounts to eight seconds longer than a year ago, according to QSR magazine and Insula Research.
In this year's ranking, Wendy's held on to its claim as the speediest chain, while McDonald's posted its slowest showing in the history of the 15-year-old study. Rival chain Burger King was the only company to speed things up.
"Driving this increase in speed of service time is these more complex menu items," said Sam Oches, the editor of QSR, which covers quick-service and fast-casual restaurants. "Consumers are demanding more fresh, upscale menu items from fast-food restaurants and as these chains are answering that demand, the new menu items take a little more time to assemble."
(Read more: Secret's out! Hidden restaurant menu items)
Added complexity from items such as McDonald's Premium McWrap or Taco Bell's Cantina Bell menu means new operational procedures and additional ingredients, which translate to a more complicated assembly line.
Compounding this problem is a busier drive-thru line. The average number of vehicles in line rose 9 percent this year to 2.82 cars. Chick-fil-A had the longest average line at about six cars while the Taco John's line was about a fifth of that.
The annual survey armed researchers with stopwatches and clipboards and sent them through drive-thrus at the nation's top quick-service restaurants. The researchers then visited hundreds of restaurant units to gather data on more than 1,800 service times during the lunch and dinner rush hours while ordering a main item, side and beverage.
(Read more: McDonald's shakes up value menu)
The study critiqued six "benchmark chains" and one regional chain, on several factors, including service time, order accuracy, landscaping condition and customer service.
To crack down on longer lines, some chains are testing mobile ordering systems and reconfiguring the drive-thru into dual lanes.
Although Wendy's quick drive-thru clocked in at 133 seconds, it's a far cry from its low of 116 seconds in 2003.
That's around the time when speed peaked, according to QSR.
"Probably the reason they were fast back then, at that time the consumer demand hadn't yet turned to more premium items," Oches said. "Back then, the focus was mainly on speed."
This was before the rise of the fast-casual restaurant, where growth has outpaced the overall restaurant industry's.
Still, accuracy has shot up from its early 2000s and late 1990s performance. Back then, only about 60 percent of some chains' orders were filled accurately. This year's average was about 87 percent, with Chick-fil-A leading the pack and Krystal being the least accurate.
So what do these longer wait times really mean for company bottom lines?
"It's hard to say," Oches said. "One theory is the slower the drive-thru, the fewer the cars that can get through and the less business you do, but I don't buy into that theory."
Still, it could be detrimental if regular customers get fed up with the slower wait times, he added.
"The experience is what's important and going into that is accuracy and customer service, because if the drive is slower by 20 to 30 seconds, customers will forgive you as long as they get a good experience," he said.
—By CNBC's Katie Little. Follow her on Twitter @KatieLittle