My printer was not slow because of its hardware. It was slow because an 8-bit board running Marlin spends most of its life doing arithmetic it cannot keep up with, so it just moves the toolhead more gently than the frame can actually handle. Klipper fixes this by doing the maths on a Raspberry Pi and leaving the board to push steppers.
The move took an afternoon. Flash the board with the Klipper firmware, put Klipper and Mainsail on a Pi, wire them together over USB, and copy a config for my model from the community. The fiddly bit was the printer.cfg, which is one big text file describing every pin and limit. Marlin hides this in compiled defaults; Klipper makes you say it out loud, which is annoying once and clarifying forever.
Then input shaping. I taped an accelerometer to the toolhead, ran the resonance test, and let it measure the frequencies where the frame rings like a bell. It picked the shaper, wrote the values, and the ghosting on corners simply went away.
[input_shaper]
shaper_freq_x: 52.4
shaper_freq_y: 41.8
shaper_type: mzv
After that I turned the speeds up, because now I could. Travel and print moves that used to provoke ringing held clean at nearly double the old rate. Same motors, same frame, same nozzle. The printer was always capable of this. It was just being asked politely by a board that could not keep up. New brain, old body, suddenly it flies.