Lunch + recess = 40 minutes.
Children are in the cafeteria for 20 minutes, then get dismissed to playground for 20 minutes.
However, it takes ten minutes for the children in the hot lunch line to get through the lunch line, get their food, and sit down. So the hot lunch children only have ten minutes to eat. If the hot lunch children find that they forget to get a fork or a napkin, have trouble opening milk carton, or something, it takes another precious one or two minutes to get help with that. It is not unusual to find that the hot lunch children only have 5 minutes to eat. Those children get an additional five minutes to eat, but the children just end up taking just one bite because they just want to go outside and play.
I know this first hand because the school allows parents to come in and eat lunch in the cafeteria with their children, and I have done this. From watching the children eat, I find that most of the food gets thrown out, and not eaten, partly due to lack of appeal, partly due to lack of time, and partly due to eagerness to go outside and play.
Even though my dd does not have hot lunch, she ends up taking longer than the allotted 20 minutes to eat.
When I introduced myself to one of the cafeteria ladies one day, she told me that she knew exactly which child my dd is because my dd is the slowest eater in the first grade. :D (The cafeteria lady hastened to say that it wasn't because my dd was goofing off instead of eating, like some of the other kids. She reassured me that my dd just plugged away at her lunch, but was just slow. And it's true, my dd IS a slow eater, and she is the fourth generation in a family of slow eaters.) The cafeteria ladies don't rush my dd when eating, they just prod those children who talk instead of eat. Since this particular cafeteria lady has seen me eat with my daughter, and has seen that I eat much slower than even my dd, perhaps she was afraid to complain to my face too much. :D