Posts like this that talk about learning "efficiency" always come off as soulless and dystopian to me. I think learning should be fun and that fun learning is the most effective---that's the only thing I optimize for and I certainly don't think about efficiency percentages. What a drag that would be.
The people I've thought were most successful were not grinding their way through learning but enjoying it, letting new questions arise and pursuing them. Is that efficient? It might not seem so. But the learning tends to be more transformative; they grok the lessons more deeply.
A work colleague once told me that it's like reading textbooks in graduate school -- you read it once just to get a general sense of the vocabulary. They you start over, concentrating on the meaning. New ideas take a lot of study to learn well. "Efficient" learning strikes me as a compromise where you get a superficial understanding for the sake of speed.