Interesting idea but I think the difficulty starts off a bit hard too soon. I never used the space bar to speed up because I always felt like I needed to slow down. Maybe players could get a higher score for finishing levels faster and that would encourage them to push a bit more.
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I was definitely rushing at bit at the end of the jam in order to create a bunch of levels, so the difficulty isn't as balanced as it could be. Especially because I was playing through the levels so much while making it, I became used to it, lol. I think if I were to flesh out this idea more, I would add some incentive for players to try and master the levels like you suggested.
Thanks for playing and commenting!