In other words, my playing of Chopins op 10 no 2 will never get faster or more accurate?
Of course it will ... but for different reasons than many people believe
If you lacked any speed and had to build it you wouldn't be able to even play two or three notes at the fastest speed possible ... and yet even non pianists can do this
This proves the point about speed. Speed is not something we lack and need to "develop" or "aquire". Speed is something we lose and need to maintain.
The contractions, the motions, the firming you do is accumulated
You can't accumulate it in just two or three notes that's why when you play such a short amount of music your natural intrinsic speed shows up.
But you accumulate them as those 4 notes become a flow of 30, 200, 500, 1000 notes
The point about maintaining speed is "resetting" periodically
When practice is making things faster you're not actually changing anything in your physiology and are not training the hypertrophy of your muscles, you're actually training your neurological resetting ... speed being the delay between contracting and resetting
Although practice itself works in this way focusing on the actual source of improvement, speed and the sources to obtain it helps with technique and control but especially with avoiding the accumulation of tension hence pain or injuries. As someone said accumulation of tension is the most detrimental thing against speed, it works like opposite friction. After all speed in piano playing is not due to bigger forearm muscles, to switching in type of fibers, to increased upregulation of concractile receptors, to stronger ligaments and muscles or faster/stronger fingers (whatever that is supposed to mean) Because these things are not happening in relation to play playing. Again it's not even slightly comparable to the effect of running on big muscles like quadriceps.
Speed is neurological and somehow illusory since again it's not something that we should "aquire" but "preserve" as the piece progresses
We have done already enough writing, grasping, picking up in our life to have already developed muscles (we are talking about small muscles and not big ones like quadriceps) already allowing the fastest speed possible.
Endurance works by the same principle
Endurance in relation to bigger muscles is the ability to use your fuel efficiently
Endurance in small muscles and in the kind of movements involved in piano playing is a matter of preventing accumulation i.e preventing opposite friction
Endurance in relation to piano playing is the difference between a ship going with the flow or going against the flow