Raised fingers in slow scale practice.
If you’ve ever listened to the great pianists play, it’s dazzling how pristine their single line fast passages are. Magical and pearl like evenness.
The solution to this is the raised fingers approach in scale practice. When practicing scales and arpeggios slowly (and you should be practicing them slowly), raise each finger above the note before striking it, then bring it down precisely and emphatically the carful attention to the exact position and timing of the stroke.
This may seem like a strange idea, after all when you play fast you don’t wan’t to be doing this. The reason this works is that these pristine fast passages are largely achieved by the releasing of notes. Yes the lifting of the finger after the note has been played. The raising of fingers in slow practice helps strengthen this mechanism, training the muscle memory to lift the fingers precisely so as not to let them drag.