Fantaisie-Impromptu, Op. 66
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Tier 3, which nobody believes until they see the chart
The Fantaisie-Impromptu has a reputation as a wall. It is 176 beats per minute, 3,050 notes in 4:31, and it peaks at 18.3 notes per second, denser than 11.7 for Clair de Lune and roughly double Für Elise's 9.2.
And it comes out tier 3 of 5, the same tier as Für Elise and Bach's Prelude in C.
The reason is what tier measures: how many notes each hand holds at once and how far it has to jump. Here the answer is at most two notes per hand, and the hands stay in position. Look at the piano roll and you can see it, two steady horizontal bands that barely move. The notes are relentless but your hands are not travelling.
Four against three
What makes it hard is the cross-rhythm. The right hand runs in semiquavers while the left runs in triplets, so four notes in one hand line up against three in the other, over and over. Neither hand is difficult alone. Holding both at once, at 176 bpm, is the whole piece.
Which is exactly what the hand-split mode is for. Play the right hand while Klaviero plays the left, until the semiquavers are automatic and you stop counting. Then swap. Then put them together, and the cross-rhythm is the only new thing left to learn instead of one of three.
Playing it here
Practice mode holds each note until you play it, which lets you take the cross-rhythm at any speed you like. Rhythm mode does not wait, and at 18.3 notes per second a dropped combo comes back fast.