The Short Order — Why does serving the quickest first make everyone wait less?

When a lot of people are queuing for one thing, the order you serve them in cannot make anything go faster - but it decides how long everybody waits, and serving the quick jobs first can cut the total waiting in half. You run the window of a waffle wagon with one iron on the last nights of a winter market, with your six-year-old brother standing at the back of the line holding his own coin, and you find out why supermarkets keep a lane for ten items or fewer.