Playing a hard piece through again and again mostly rehearses stumbling in the same places. What works is making a short stretch you can play correctly at a slow tempo, repeating it, and raising the tempo a little at a time. The player here has the two tools that takes: playback speed and the loop.
Playback speed and note speed are different things
Playback speed is the selector marked "1x" on the control bar, from 0.25x to 2x in steps of 0.25; the up and down arrow keys move it one step at a time. Slowing down does not change the pitch.
At the top left you will see the tempo (for example "120 BPM"), the "Rate:" multiplier and the "Hi-Speed:" note speed. The BPM is the piece's opening tempo multiplied by the rate, and it does not follow tempo changes later in the piece; treat it as a reference for setting your own metronome. Hi-Speed only changes how fast the notes appear to fall, never how fast the music goes.
Setting up a loop
The loop button on the control bar turns looping on and off. With no range chosen, the whole piece repeats. To repeat one passage, open Settings, tick Enable Loop under Loop Range, and set the passage with the Start: and End: sliders.
Hovering over the seek bar shows the time at that point, so check the times either side of the passage first and then set the sliders to match. The chosen range is highlighted on the seek bar and shown at the top left as, for example, "Loop: 0:12–0:20". The R key turns the loop on and off.
Choosing what to loop
Keep loops short: one phrase, or one trouble spot, a few seconds to perhaps fifteen. A long loop lets your attention drift away from the hard part on every pass.
Start the loop a little before the hard spot - a beat or so - and end it on the first note after it. That way you practise getting into the difficult notes and getting out of them, not just the notes themselves. Cut out the hard spot alone and you tend to end up with a passage that works on the loop and falls apart in context.
Once a loop is steady, move it on so the next one overlaps it slightly, and finally check the two joined as one longer loop. Working each hand alone first, following its colour, and only then hands together works well here too.
Bringing the tempo up in steps
Start at a speed where you can play the passage correctly without stopping - often around 0.5x. Decide on a rule for moving on, say three clean repetitions in a row, then go up one step: 0.5x, 0.75x, 1x. If it falls apart, go back a step.
The step from 0.75x to 1x is bigger than it looks. If the passage breaks down there, do more repetitions at 0.75x, go back to hands separately, or shorten the loop. The next day, start one step below where you finished; your fingers warm up on the way back to the tempo you reached.
Note speed when you slow down
At 0.5x the notes simply fall more slowly; the bars keep the same length and spacing, because the spacing is set per second of music, not by the playback speed. If a run is hard to separate, raise the note speed (the comma and full-stop keys, or Note Speed in Settings) to spread the bars out. Lowering it packs them closer but fits more of what is coming on screen, so you can read further ahead.
When you slow a piece from the site's collection down or use the loop, the practice record you save afterwards comes with "Slow" or "Section" already ticked. Noting where you practised and at what speed makes the next day's starting point easy to choose.