> It only takes days to get twice as effective at reading with spaced repetition? Or at learning? If it's the latter, I don't think that's what the earlier study measured?

It only takes days for spaced repetition to get twice as effective, not reading. It takes you years to get twice as effective at reading.

> I think about other ways to build your understanding, like working through problems and applying the knowledge, that are key to learning. Much of learning physics is getting your hands dirty in the equations, and there's a big difference between knowing a formula and really understanding it in action.

I agree, you should definitely work through problems and apply your knowledge. The post is arguing that you should do that AND spend a little bit of time doing spaced repetition, not to only do spaced repetition and nothing else.