The study method that works for you is not necessarily the best for everybody. It is nearly always a trade off between the time you have, the ability you have, and the mark you need to pass. Part of the evaluation process is to see if you can absorb the material in the time available.
Take a look at the time you have spent doing a particular kind of studying. How does that seem to be proceeding? Are you retaining the information? When you look back at material you studied, say, 1 month ago, are you still able to remember most of it?
Some people have excellent short term memory. They can load up a subject just long enough for the test. Then in six months they have forgotten most of it. It means they can pass tests. Maybe they can do certain kinds of work. Maybe it's a problem if they can't get the important stuff into their long term memory. Or maybe it's just fine if they can do 15 minutes of refreshing, and be ready to go.
Personally, I find that I don't really know a subject until the third time I have gone all the way through it. And I mean in globby equation-filled detail. I have to derive every stinking little result and detail three times to be able to retain it. No cheating and looking at my old notes, but doing everything cold. It has meant that I missed a few nights sleep to pass tests. But mostly I retained the stuff that I went through this ordeal over. And it made me a terror on tests. I was nearly always spoiling the curve for everybody else. Because that 2nd and 3rd time through pushed it through to my long-term memory. So in 2nd year I was able to remember the 1st year stuff without having to search through the texts.
Maybe this is not for your. Maybe there are more efficient ways. I find, for example, that learning a new language is a very difficult thing for me. I have not retained much of the French I was required to take in school. And learning Mandarin to please my gf is a slow painful thing. Ni hao. Ni hao ma?
In undergrad, I had a prof who seemed to know the subject cold. He could fill 12 boards full of equations while barely referring to his notes. Which were on the desk on the other side of the room. We joked we needed water cooled pens to keep up with him. But then, maybe that's because he had taught this course 10 times previously. Maybe it's a possible way to learn a subject, by preparing lecture notes on it. Write it up as though you were going to explain it to somebody at the level you were when you started the class. Beware of how long such a project might take!