Your assumption is that they were doing 5 days worth of work in 5 days, and now they're doing 5 days worth of work in 4. But they're just more productive, they're not busier. So it sounds like before they were doing 4 days worth of work in 5.

I think it really depends on what the work entails. When I’ve worked on some particularly complex problems it’s often the case that I get “zero” days’ work done for four days, then “dozens” of days’ work done in one. My assumption is that a person has a fairly fixed productivity capacity over a given stretch of time, that we as a society tend to organize that as a week of seven days, and that it’s highly likely most people (again depending on the work and the person) reach that capacity in less than five days.