I like the observation about the importance of concentration.

In addition to reading, I've noticed that studying mathematics is an opportunity to practice (or build) the ability to focus on frustratingly difficult material that requires a high attention to detail.

People talk a lot about programming being similar to math, programming ability being similar to mathematics ability. I've always been a shitty symbol-pusher and a reasonable programmer, so I thought this was a bunch of bullshit. Then I actually took a post-calculus math course (Intro to Analysis and Linear Algebra). The material was irrelevant but the cognitive muscles exercised were very similar.

I actually really like this hypothesis. There is definitely a distinction between long-form content and short-form content, and programming certainly lies in the former.

That's not to say that short-form content doesn't have its place, I'm pretty sure I would learn less if I didn't browse Reddit or HN during lunchtime, and for some reason I feel like reading a book wouldn't fit in that 15 - 30 minute period. But it's still a period I can put to use.

dual n-back is questionable in terms of improving iq / working memory, but if you want to try something much more concentration intensive than possibly anything else, give it a shot

brainworkshop seems to be the most popular tool