An excellent question. For the first couple years at Caltech, I struggled a lot. The second two were easier, though I was learning more material faster.

I think what happened was I "learned how to learn". Part of that indeed was getting better at picking out what to focus on. I don't believe I got any smarter.

It was still a full time job to keep up, though. 33 years after I graduated, I still have "examination dreams" where I attend a class and have no idea what the prof is talking about. Fortunately, these dreams have become less and less frequent over the decades :-) But it was worth it. I'm glad I've never been in combat, I shudder to think about the nightmares vets must suffer.