Elon's technique makes sense, but you can't just do that. The most impressive people seem to be T-shaped: they have very deep understanding of their specialty, but they also a familiarity with a wide breadth of fields.
I skim a bunch of technical software books that are not immediately useful to me. My goal is not to remember every detail, it's just to get an idea of what's out there, and where to find more information if I need it.
"One bit of advice: it is important to view knowledge as sort of a semantic tree -- make sure you understand the fundamental principles, ie the trunk and big branches, before you get into the leaves/details or there is nothing for them to hang on to."
-Elon Musk Reddit AMA https://www.reddit.com/r/IAmA/comments/2rgsan/i_am_elon_musk...
Should I read slowly and be well read? Or read quickly and get the big ideas of lots of fields?
Or rather double/triple/quadrouple/n-ple tee-shaped having read broadly across many fields and comparatively deeply in more than one. Relative depth in multiple areas becomes easier and more likely with age as interests change and realized opportunities to read to greater depth accumulate.
Reading deeply requires time. Reading broadly requires time. Neither has a shortcut. Neither precludes the other given a lifetime.