Just want to comment on (paraphrasing), “you have to fight the things that are in your hands”.
At slow speeds the brain can play each note, at higher speeds the brain starts to work in gestures and depends on “the hand” to execute larger chunks of notes.
So if you want to improvise at fast tempis, what you want is some kind of automatism. The key is you want open (as opposed to closed) automatisms. Closed automatism is what happens when you’re able to play say a bach piece at high speed. If you change a single note or a fingering, chances are the whole thing falls apart. Open automatisms is what you use when driving a car, you don’t have to think about changing gear, it’s automatic, but it only happens when you want it.
The way to achieve open automatisms is to practice as many variations of ‘something’ as you can possibly think of, shift it around, start on all beats, remove first note, remove first two notes, vary the ending, practice if coming from another idea, all 12 keys.
So in my opinion you shouldn’t fight what’s in your hands, but rather take control over what gets to be in your hands, and make sure it’s super flexible.