If this were really true, you’d expect to see most people with full freedom to set their own hours gravitate towards that 30-hour mark.

Professors working 30-hour weeks would write more papers than their harder-working colleagues. Business owners would show up for 30 hours a week and go home. Fruit pickers, paid by the bucket, would pick more fruit in 30 hours than their colleagues could pick in 50. Professional athletes who spent less time training would beat their harder-working competitors.

It's an idea that's only recently begun gathering research:

https://www.entrepreneur.com/article/294545

https://news.ycombinator.com/item?id=13318990

https://www.brinknews.com/working-fewer-hours-makes-you-more...

> Professors working 30-hour weeks

This is gated by the fact that teaching even a single class well is almost a 40-hour a week job by itself.