I do not see it mentioned in the comments anywhere, but the method that has worked for me, especially for hard subjects, is to start by reading a few books and get a "macro" view of the topic.

Example: Right now, I'm trying to learn a lot about quantum mechanics, so I read two books and learned about the standard model, Schrödinger equation, the three generations of matter, relativity etc. I try to determine how they all fit together. I don't draw memory maps, but I basically try to calculate one in my head (probably should just draw them). Then I stick all of those concepts + new vocabulary into anki and review them daily. After a month or two, I pick up another few books and get deeper ("micro" view). When I get bored, I stop and move on to a new subject.

I have done this with microbiology, physics, some time periods of history, computer science, etc. You just need to read a lot. I do not think speed reading matters but skipping over parts of books that are not relevant will certainly save you time.