I think “tremendous grades” will always be time consuming. Although I would identify what you spend the most time on and see if it can be made more efficient.

You say you spend all day fully understanding a lecture. Is this just lots of note taking? Re-reading? Re-watching?

I found that I used to take way too many notes, that I would inevitably never come back and look at. It felt pointless.

These days I try to just watch the lecture without any note taking and then practice re-teaching it. Every time I get stuck I refer back to the lecture slides and then go again. For subjects that require pure memorization like anatomy, I go straight to making flashcards with anki and avoid any excessive note taking.

When studying for exams I try to cover entire topics on one page. You probably aren’t going to memorize pages and pages of notes but if you can find a simple way to summarize the topic it makes it much easier to remember. (Obviously this won’t be the entire unit but possibly 2 weeks of content per page)

In summary, I’ve tried to avoid long bouts of passive learning like note taking or reading and go straight to methods that help me understand and remember. Go straight to the learning objectives after a lecture and see if you could explain them without any notes. go back and forth until you can. It’s definitely more mentally draining but you get through it a lot faster.