Étude Op. 25 No. 11, "Winter Wind"
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One hand does two thirds of the work
3,172 notes over 3:07, peaking at 20.6 notes per second. The number that tells you what this piece is, though, is the split: 66 percent of those notes are in the right hand.
That is the most lopsided distribution of the twelve pieces with a page here. Compare Clair de Lune, which is close to an even split, or the Turkish March at 54 percent. Chopin wrote this as a continuous chromatic gale in the right hand over a march in the left, and the arithmetic says exactly that.
You can see it in the piano roll without being told. The cyan band across the upper half of the picture is dense and unbroken. The magenta underneath it is sparse and regular, a steady tread. Two completely different jobs, played at the same time.
Why the hand-split mode matters more here than usual
On most pieces, learning one hand at a time is a convenience. On this one it is close to the whole method, because the two hands are not two halves of one problem. The left hand is playable early. The right hand is the étude.
So take the right hand alone and let Klaviero play the march underneath. You get the harmonic context you need to hear whether you are in the right place, and you spend all your attention on the only part that is hard. That is not a shortcut, it is what an étude is for.
The tier is 4, not 5
At 20.6 notes per second Winter Wind is denser than the Fantaisie-Impromptu and nearly as dense as Flight of the Bumblebee, yet it lands at tier 4 rather than 5. The chords get large but the hands do not have to leap between positions the way La Campanella does. It is fast, not scattered.