thanks for posting the demo!
I agree with Pandan3d's comment about the camera.
I had mixed feeling about hold to double jump. it feels like it could be a regular double jump. it might have some merit in certain situation involving high momentum, but on the slow section where it's introduced it kinda got in the way.
some grapple section are very easy to miss by going too fast or jumping too high, though correcting your course with mid air boost is pretty fun.
overall, I'm happy to see a speedy 3d platformer and would keep an eye on its development!