Not really sure how to do it best, main problem with my keyboard is i can't jump unless i released S, so it added to dificulty of timing inputs.
Maybe only allow the fast fall during a jump? so you don't fall straigh after a platform if holding S. That would mean disabling it when touching ground, and allowing it again when jumping. That may cause some others problems elsewere, not sure if fast fall is needed anywhere without a jump.
I finaly completed the game after around 20 minute spent at the last 2 checkpoints, Maybe the difficulty should grow more smoothly. The last 2 checkpoints are very hard( in a good way, every attempt is better and feel easier than the previous one), and since it's the first time we face those kind of jumps, we have to learn them from the difficult section.