Prelude & Fugue in C, BWV 846
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.
High note count, low density, which is the profile you want
1,284 notes over 3:46. That is more notes than Für Elise's 1,041 and more than Gnossienne No. 1's 755, and yet it peaks at only 8.8 notes per second and averages 5.7.
The reason is what the prelude is made of: broken chords, one note at a time, evenly spaced, all the way through. A lot of notes arrive over the course of the piece and very few arrive at the same moment. That combination, plenty of notes and no density spikes, is close to the ideal profile for a first classical piece, and the piano roll is the proof. It is a regular repeating staircase. There is no dense patch anywhere in it.
What that gives you
Every note is worth the same amount of attention, so the piece teaches evenness rather than heroics. There is nothing to fake and nowhere to hide a rushed bar, which is exactly why it has been the first page of keyboard study for three hundred years.
At tier 3 of 5, the hands hold at most two notes each and stay in position. It sits in the same tier as Für Elise and the Fantaisie-Impromptu, which is a good reminder that the tier measures shape rather than speed.
The fugue is the other half
This chart is the prelude and the fugue together, which is why the note count is higher than a recording of the famous prelude alone would suggest. The fugue is where the piece stops being a study in evenness and starts being three independent voices, and on the piano roll it is the section where the neat staircase gives way to lines crossing each other.
Playing it here
Practice mode holds each note until you play it, which is the right way to build the evenness this piece is for. Then switch to Rhythm mode, where the regular spacing makes a 25-note combo straightforward to reach and a single rushed note obvious.
BWV 846 is in the free tier.