I doubt whether teachers in most countries worry so much about whether students retain 5% or 95% of the lecture. You know why?
Because most every industrialized country (and non-industrialized country for that matter) has real competency based standardized tests that the students know they must pass to proceed.
Instead, we fret and wring our hands about whether our students are learning. (And this is very useful.) But it's amazing how little effort we put towards "doing business wholesale" which means setting up a system that will motivate students to learn however the material is placed before them. In these other countries, you learn or else and the teacher stands on the same side of the fence to help his/her students.
Discussing these micro issues in America is like Roger Penske debating the efficacy of a new carburetor on a Lola with flat tires.
Until we light a fire under American kids like they do in most every other country, we're only rearranging deck chairs on the Titanic. And BTW, you would see teachers really trying to improve their teaching if real student and teacher accountability was instituted.
If you doubt my position, contemplate how much extra time you put into your AP kids. There's a real test, real consequences (but even in this case, the down side is pretty soft).
Let's get real about improving American math education. Doing everything "retail" ain't gonna' make it.
Dan Hart San Fernando H.S. LAUSD "The other absurdity near Bunker Hill"