As a former McDonald's manager, it is STORE POLICY to pull you ahead if YOUR food is not ready. If the person behind you just ordered a coffee and you're waiting on a chicken, it makes the line go A LOT FASTER if you just pull up so OTHER PEOPLE can get their food.
That being said, chicken takes a longer time to cook than the beef or fish. Fried, it takes about 6 minutes, grilled about eight. We used to have bank employees come in and order five salads all with grilled chicken and they would usually have to wait. They did not come at regular intervals like every Friday, they would come whenever. Sometimes there would be two employees, sometimes five. If you happened to get in line right after something like that, then the employees would be waiting for the new chicken to cook.
We once had a customer refuse to move ahead, so we began to run food to the back drive thru and hand food out there.
Fast Food is a very stressful job and I have seen adults with college degrees buckle down and say "I can't do this!" In a way, I kind of miss it because I was very lean from constant moving, I gained weight when I left.
I have all sorts of stories about stupid or rude customers (or sometimes just amusing) and if people were somewhat polite things would go a lot easier. We don't run out of chicken on purpose during busy times, we just want you to get your food and move on. Only once in 6 years did I ever experience one of our employees being purposefully rude to a customer (and in my co-worker's defense, the woman was rude to her first and excessively so. The woman never complained because there were too many witnesses that saw her throw straws at my co-worker)