Prelude Op. 28 No. 15, "Raindrop"
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The longest gentle piece in the free tier
1,518 notes over 4:31, peaking at 8.3 notes per second and averaging 5.6. That puts it in the gentlest quarter of the library while still being four and a half minutes of real music, which is a rarer combination than it sounds. Most pieces this undemanding are also short.
The name comes from a single repeated note, an A flat, that runs through nearly the whole prelude as a steady pulse. It is the clearest thing on the piano roll: a flat horizontal line that almost never breaks, from the first bar to the last, with the melody moving above and below it.
Its own opus contains the counter-example
Chopin wrote twenty-four preludes as Op. 28, and they are not a graded set. Three along from this one, the Prelude Op. 28 No. 12 is 1,243 notes packed into 0:57 and peaks at 27.2 notes per second, more than three times the Raindrop's 8.3 and the fourth densest chart in the whole library.
Same composer, same opus number, published together. If you have ever assumed that a piece is at your level because you can play its neighbour, that pair is the reason not to. Op. 28 also runs the other way: No. 20 is 286 notes at 4.5 notes per second, gentler than the Raindrop again.
Playing it here
The repeated note is in whichever hand is not carrying the melody, and it swaps. That makes the hand-split mode genuinely useful: play the melody hand and let Klaviero hold the pulse, so you can hear where the swap happens instead of counting it.
Practice mode holds each note until you play it. In Rhythm mode the steady pulse is actually helpful, because a combo of 25 arrives quickly when a note lands every 74 beats per minute without a gap.
The Raindrop Prelude is in the free tier.