Clair de Lune
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 clearest case in the library of density and difficulty coming apart
Clair de Lune is 1,491 notes over 4:08. It peaks at 11.7 notes per second, which puts it in the gentler half of the library, and its average is only 6.0. The peak is nearly twice the average, so most of the piece is considerably quieter than that top figure suggests.
And it is still tier 5 of 5, the hardest tier we have.
That is not a contradiction, it is the limit of what density can see. Density counts how many notes arrive and how fast. It cannot see how far apart they are. Clair de Lune is tier 5 because of chord size and the stretch between the hands, not because the notes come quickly. The piano roll above shows it: the notes are spread across a wide vertical band, low left-hand notes a long way under the melody, and that distance is the work.
What that means for practising it
The hands are almost evenly loaded, 1,491 notes split close to half and half. That is unusual, and it means neither hand is the easy one. Learning the right hand first does not buy you as much here as it does in Winter Wind, where two thirds of the notes are in one hand.
At 100 bpm with a peak of 11.7 notes per second, nothing in it will outrun your fingers. Play it in Practice mode until the chord shapes are under your hands, because that is what the difficulty actually is, then switch to Rhythm mode for the timing.
Playing it here
Right hand cyan, left hand magenta. Play one hand and Klaviero plays the other. Rhythm mode judges every note PERFECT to MISS, runs a combo to ×6 and drops you into FEVER at 25 in a row.
Clair de Lune is in the free tier.