This is universal. I’ve had largely the same experience. There’s several reasons for this.
1. Stupider people are better teachers. Smart people are too smart to have any empathic experience on what it’s like to not get something. They assume the world is smart like them so they glaze over topics they found trivial but most people found confusing.
2. They don’t need to teach. If the student body is so smart then the students themselves can learn without teaching.
3. Since students learn so well there’s no way to differentiate. So institutions make the material harder. They do this to differentiate students and give rankings. Inevitably this makes education worse.
It's simpler than that. "Prestigious" universities emphasize research prestige over all else on faculty. Faculty optimize for it and some even delight in being "hard" (bad) teachers because they see it as beneath them.
Less "prestigious" universities apply less of that pressure.
It can also be different within the same university, by department. I graduated from a university with a highly ranked and research oriented engineering department. I started in computer engineering which was in the college of engineering but ended up switching to computer science which was in the college of arts and sciences. The difference in the teachers and classroom experience was remarkable. It definitely seemed like the professors in the CS department actually wanted to teach and actually enjoyed teaching as compared to the engineering professors who treated it like it was wasting their time and expected you to learn everything from the book and their half-assed bullet point one way lectures. Unfortunately or fortunately, depending on your view, it also meant having to take more traditional liberal arts type electives in order to graduate.
I did once have a Physics lecturer say " When I took Quantum Mechanics back in my undergrad, I got an A but didn't actually understand anything" and then in the same lecture 20 minutes later: "What part of this do you not understand?" when the entire class was just blankly looking at the whiteboard.
At least at the undergrad level, it's not impossible to get an "A" without actually learning anything. Especially Freshman/Sophomore level classes. You just cram for the exams and regurgitate what you memorized. Within a few months time it's mostly gone.
Seriously, what so non-understandable in first 20 minutes of QM?
Probably depends on how it’s explained, no?
I could make arithmetic incomprehensible, let alone QM.
They never implied it was the first 20 minutes of the entire course