I notice this especially in the arpeggio passage of the Solfeggio in C minor. But when I am playing very fast arpeggios, the notes either stay separate or blend depending on the dynamic. At forte, 16th note arpeggios at Presto will stay separate and I hear individual sixteenth notes. At piano however, those same arpeggios blend so much that it goes outside of the 1 e & a pattern of sixteenth notes and sounds more like an arpeggiated chord than anything else and sounds like it is closer to 32nd notes in terms of note speed.
Why is it that my hands play the arpeggios so much faster at piano than at forte? Is it because I put so much more force into pressing the key when I do forte arpeggios that less of the force of my hand contributes to the motion of the arpeggio and so when I play arpeggios at piano, almost all of that force contributes to the motion of the arpeggio and thus I tend to get way faster arpeggios at piano than I do at forte?
This makes it harder to play the passage right when I am not using a metronome where I have each tick be a 16th note in length. But I mean, I eventually have to be able to do it without the metronome, because what if I have to play a Presto piece with lots of arpeggios at a concert or something? While this might not be so bad for a piece by Chopin where rubato is expected, what if I'm playing a Beethoven piece and I am expected to stay in the meter exactly until I see accelerando or ritardando?
So yeah, why do my arpeggios at piano tend to be so much faster than the same arpeggios at forte even though in both cases I try to stay within the tempo and not go faster?