> Imagine you give two people the same task and ask them to complete it in one week. One person works on it four hours a day, the other six. They both get it done on time, at the same level of quality. The first person was actually _more_ productive than the second.

I'd argue that they were equally productive. To be more productive, they'd have to produce more work product during the week.

Especially if it's a primarily mental task, there's a good chance that the 4-hour / day person is actually making progress when they're not visibly working, which would interfere with any attempts to either move the deadline earlier or get other work done in parallel.