I have been able to play super fast(presto) once but that was because I practiced a sonatina at allegro for weeks and presto isn't all that far from allegro.
I have also been able to play really fast notes at allegro but I know that fast notes and fast tempo are completely different things.
This is the first movement of K 545 and it has a lot of really fast notes at allegro. This is easy for me.
This however(solfeggio in C minor) I can't get to presto without at least 1 mistake. Presto in general is hard for me unlike molto allegro.
Here is how I practice fast pieces(allegro and above):
Start adagio with 1 hand at a time. Keep practicing until there are no mistakes in either hand. -> Same tempo but with both hands. I ignore things like crescendo, accelerando, and poco a poco and just play the whole thing mezzo forte at a constant tempo. -> Gradually speed up and once I get mistakes stop at that tempo and practice. Usually it is like this: Andante -> Moderato -> Allegro -> Start making it more expressive(this is when I do the crescendo, accelerando, poco a poco, and other things to make it more expressivo).
Now normally the fastest pieces I learn are in the allegro range and I am able to play them well in just a few weeks or months depending on the length of the piece.
However going from allegro to presto is what keeps me at allegro. Unlike going from adagio to andante or moderato to allegro where I have fewer mistakes, going from allegro to presto I have mistakes just about everywhere. Even if I can do it perfectly at allegro, when I speed it up to presto I just can't do it.
The same thing is true if the accelerando is going up to presto.
How can I keep this allegro to presto transition from stopping me at allegro? Is there a method I can use to successfully go from allegro to presto whether presto is the tempo of the piece or it is an accelerando in an allegro piece?