If chang were alone in predicting speed walls, I might agree with you.
But he is not, and slow incremental progress does not get you to the next level.
As I tried to say before, you are not understanding my post. Many actions in playing an instrument are actually a series of sub-actions, directing of energy, an efficiency that can get trained in layers. We can blur it all into one motion, especially when going fast, and limit ourselves into something inefficient. There are also ways of using and experiencing the body in ways we cannot imagine until we have done so. "Slow practice" and getting faster, while still doing the old things, will not do it. That's why I wrote about good teachers, knowing how to follow them and such (at least I hope I did).
The playing of a scale is actually a complicated manouver. You have your hand in a particular position, fingers already touching the keys. In Cooke's method, which I've also been told by a good pianist/teacher, with the right hand, the minute you have played 2, your thumb scoots under at the joint at the wrist (where and how matters) ..... exercises to give flexibility and the correct motion precede this ..... so that when you have finished playing 3, your thumb is already there for the 4th note. At the moment that 4th note has been played, almost simultaneously but in fact, afterward, your hand moves into the next position so that the four fingers that come after the thumb again rest comfortably on the respective keys. When your hand moves over your thumb is also unbending at a particular joint. So now your four fingers are in their position and they are ready to play - as soon as 2 is played the same thing happens again, except that this time the thumb has a greater distance (needs the flexibility, moving from the correct joint) because it's reaching for the tonic.
These separate motions are choreographed, trained into the hand, brought together. When the fingers are on their respective keys, instead of reaching for them mid-flight, you get rapidity and ease. You are training all that while you are going slowly, making sure you don't lose it as you speed up. At the end you have a whisper of a motion, but the efficient way of play is there.
An antithesis: I played something that I have not played for a year and had tension in my hand, inability to play quickly or do the dynamics I wanted. That is because now I know that I should keep my hand small for playing close together notes, and expand it when playing a greater expanse of notes. There is a deliberate moment of expanding or contracting the hand when going from one configuration to the next. In the past my hand was ready for whatever note anywhere, and so in a constant state of tension. I was doing it all at once - by slowing down enough to get the different hand shapes and positions, I was actually speeding myself up.
Slow playing is not a matter of playing slowly and passively thinking it will settle into our memory. The reason we play slowly is because there is a lot that we want to consciously put into our playing, and there isn't enough time when we play fast. Later on these things are automatic - subtle little movements make all the difference.
Topic: Chang and frustration