I have trouble getting students to concentrate on difficult material for anywhere near a whole class even if it's their only class of the day (and I'm considered a good and entertaining professor, honest). No WAY I could get anyone to concentrate on difficult material for 40 hours per week. I mean really no way, even if I were a teaching genius.

Learning difficult material means taking breaks and digesting stuff subconsciously.

Therefore, learning anything difficult takes a certain minimum of calendar time as well as hours in the classroom.