Un Sospiro
You need a MIDI keyboard and Chrome, Edge, Firefox or Opera on a desktop or laptop. Not Safari, not iPad, because Apple does not support Web MIDI. Full requirements.
The densest chart in the library
Of all 315 pieces we have measured, Un Sospiro is number 1. It peaks at 31.4 notes per second.
It is worth being concrete about what that figure means, because a number like 31.4 stops registering. The measure is a rolling 25 second window. So somewhere in this piece there is a stretch of 25 seconds containing 785 notes, with no bar to breathe in. That is the busiest 25 seconds of music in the entire library.
The average across the whole piece is 16.7, a spread of 1.9 times, so unlike Flight of the Bumblebee this one does have quieter ground. It just also has that window.
What the hands are actually doing
The title means a sigh, and the piece is built on an arpeggio that runs continuously underneath a melody, with the hands crossing over each other to keep the melody sounding while the arpeggio keeps moving. The melody is not in one hand. It moves between them.
That is visible in the piano roll: the cyan and magenta bands overlap and swap places rather than sitting one above the other, which is what hand-crossing looks like when you plot it. Compare it to Winter Wind, where one colour plainly owns the top of the picture.
It covers E♭1 to F#7, so both hands travel a long way while they do it.
Playing it here
Take one hand at a time and let Klaviero play the other. On a piece where the hands cross, that is not just a practice aid, it is the only way to work out which hand is supposed to be where without guessing.
Practice mode holds each note until you play it, so the arpeggio can go as slowly as you need. Rhythm mode does not wait, and at 31.4 notes per second the combo will break before you have finished reading this sentence.