They will usually be picky about accepting students, won't pass anyone who doesn't earn it and tend to take substantially longer than the agency minima to finish courses. I believe
@NetDoc and
@boulderjohn are in this group among others.
I have done almost all (but not all) of my OW instruction while working for a dive shop, which means I have the same pool time and classroom time as all the other instructors. With me as an instructor while working for such a shop, my OW courses did not take any longer than anyone else's. But I do teach students to be neutrally buoyant and in horizontal from the very first time they are in the water, so by the end of class my students will invariably look like seasoned divers. They do all their later skills while hovering in mid water, including weight belt removal/replacement and scuba unit removal/replacement. They don't have to touch the bottom to remove and replace a mask. The most experienced instructors in the shop I last worked for have much more experience than I, and they refuse to make this change. They still teach the students while on the knees and grossly oveweighted, so their students have very poor buoyancy and trim when they are done with the class. It therefore looks as if my students are taking more time to finish, but they are not. Students learn the initial skills more quickly when they are in horizontal trim than when on the knees, so those parts of the class go much more quickly. This gives me more time to have students simply swim about while getting used to neutral buoyancy diving than the other instructors have for their students.
But I pretty much stopped doing OW instruction this year and stopped working with that shop. I am doing almost nothing but tech instruction now. I have no idea how much time my tech classes take in comparison to most other instructors. I am pretty strict abut teaching to all the standards, but it does not take students all that much time to meet those standards.
The fact that students take longer to get through your class than someone else's class does not mean it is a better class. As an obvious example, let's take the academic portion of any class. Old-timers rave about the good old days when students spent weeks attending lectures in which they received the information for a class. Lecturing is the absolute worst way to convey information to a student. It takes much more time than home study with a well-constructed instructional manual, and students who do that home study will remember much more in that lesser time. Better still is an online program that uses multi-media, built in knowledge checks, etc. That goes even faster than home study with a manual, and students have greater retention still. Taking weeks to learn less is not better than taking a couple days to learn more.
The same is true for skills. If you do a lousy job of presenting and demonstrating skills, it will take a very long time for students to master them. If your presentation and demonstration is well done, things go much more quickly. I had some experience with a tech instructor whose instructional sequence went like this:
1) Name a skill, have the student perform it, and then evaluate that performance
2) Mock that performance and tell them how crappy it was
3) Have them try again.
4) Mock them again--maybe get angry about continued incompetence
5) Sigh deeply and then tell them how to do it correctly
6) If really desperate, do a demonstration.
In explaining why he did it that way, he said that in tech diving, he does not "hold your hand" the way PADI instructors do in their OW training. It took a very long time to complete his tech classes. In contrast, I do explain and demonstrate skills as needed,so it can take you less time in my class than in that instructor's class.