Comment by DHRicoF

The cost is in the context switching. Throw 3 tasks that came 15, 20 and 30 min later. The first is mostly ok, you finish by hand. The second have some problems, ask for a rework. Then came the other and, while ok, is have some design problems. Ask another rework. Comes back the second one, and you have to remember the original task and what things you asked for change.