What the author calls the "fast growth phase" - I call "walking the tightrope and/or knife's edge". Our minds are in a constant state of attempting to optimize away any task. This is why there's a huge difference in skill levels between a person who has driven a car for 5 weeks vs 5 years, but nearly zero difference between somebody who has driven a car for 5 years vs 50 years.
The CHALLENGE is in constantly forcing your mind back to the knife's edge - and it varies from person to person.
Back when I was a highschooler, I was unsatisfied with my sheet reading capabilities. To fix it, I
- started playing only new sheet music
- grabbed a old hymnal (from the 1950s) which are rhythmically simplistic but usu. have four voices.
- Wrote a very simple app that generated random sheetxml
- Borrowed a friend's sheet music for the organ (which has three staves) and tried to transpose it on the fly to play with just the two hands on the piano