Level 3: Boredom
We have a new measure, boredom. It takes time for boredom to build up as you stay focussed on a single task, but when it does, efficiency starts to drop. If you shift between tasks, boredom remains low, but so does efficiency.
You may notice that boredom follows efficiency, just a little bit behind. This delay is where you are at your most productive.
Remember this is a simplified illustration, and of course it is possible to do something you care about for a long time without getting bored.