It's the same around here. I don't know how much of it is "business is booming" though, and suspect it has a fair amount to do with "everyone who works there is incredibly slow and incompetent".
Every time I go to a MCD's, it's like hitting a ? block in Mario Kart because I never know what I'm gonna get. Around four out of five times (not exaggerating) they get my order wrong, sometimes completely wrong, and I usually don't get charged any differently because they're done at separate windows. So I might order $5 worth of stuff and then when I get it there's an extra $5 worth of stuff there for no reason. For example today I ordered a medium sprite, got to the second window and they gave me a medium sprite + a large sprite. It is seriously a complete random grab bag when I order there. I'll find fries and other things that I didn't order, or something I did order will be missing, usually replaced by something else. The good news is that the errors are usually in the positive, i.e. I get more than I'm supposed to, or an even amount that's just wrong. Rarely do I get less.
Chick-fil-A on the other hand is tremendous and has never once gotten my order wrong, plus the food is a lot better. Unfortunately the closest one is 30 minutes away so I don't get over there very often. I'm not sure why there's such a huge disparity between CFA's rate of getting orders right and MCD's. The staff of CFA is entirely white people and the staff of MCD is entirely hispanic people, on an unrelated note. Either way I don't really mind MCD's wild incompetence because it's fun to see what I got this time whenever I look in the bag.