I'm running on Windows 10, and I'm not getting any performance issues with the game.
Thanks a ton for replying! Right now, the movement physics are rigidbody simulated. I've done what I could in the 48 hours to make it as predictable as possible (using continous rather than discrete simulation for example), however I've noticed that in high velocities it can feel pretty unreliable. When I take the game further starting monday, the physics will be completely rewritten and custom this time, rather than rigidbody-based.
That said, the game _is_ very difficult, however it should definitely not be impossible. Level 7 is a difficult jump, but both me and my playtesters nail that level pretty much every time now.