Fast food have simple menus – combo 1, 2 or 3?
but when they complicate things by asking if you want to upsize, or if you customise your orders by changing the coke to sprite and order just a burger, you slow things down. compare this to a chinese mixed vegetable rice stall where everything is already prepared, you just need to choose what you want and it gets piled on a plate of rice for you. you don’t even need to wait for them to prepare that burger that is not on their best seller list, and thus they do not have a pile of that burger already prepared.
eating it is not very fast also. kfc must be eaten sitting down, you can’t walk around and munch as you go. same as veg rice.