i find single finger repeats incredibly fast, now, for scarlatti. in taking grad piano lessons, i learned you don't have to change fingers. you give set points for as many notes are to be repeated and move incrementally inward on the note. try it. you'll like it. and you can take advantage of the double escapement and use the 'inbetween' portion to just repeat notes quickly. i can still use this technique on my kawai upright - and i like it better than switching fingers now for scarlatti anyway.ps my reason for moving inward is the mechanics of the hand and the piano key work together. but, if you move outward - you are pushing down more and you have more distance (so you get finger slap).
Repeated notes always can be controlled as a group of repeated notes controlled by a continus dropping pattern of the hand. Like in the Ravel's Alborado del Graciaco when the Rh first comes across the repeated notes it is clearly defined as groups of three. These groups of three notes have a paticular sound associated with them, different to groups of 2 repeated notes or any other. Our fingers are kept close together, they all extend and raise as the first finger drops onto the first note of the group, then the rest of the fingers repeating that note continue to drop down with gravity. If we try to strike the note with individual finger movements we are just going to tire ourselves out very fast and have a hard time controlling the sound produced.The piano you are on definatly effects how easy/hard it is to repeat notes. I find electric pianos with no touch sensitivity and instant key sping back action are the easiest instruments to repeat notes.