Thank you for this, this is the longest piece of feedback I've gotten so far.
First, I find it interesting that you call it slow, personally I think the player is a bit too fast. Can you elaborate on what makes it feel slow?
1. Interesting, the player already uses an acceleration curve with a very sharp peak at very low speeds- maybe it should be higher.
2. I'll definitely look into that.
3. True. I think some of this needs to be fixed with code, and some of it with animations. For example the mach turn shouldn't just instantly adjust your speed but accelerate toward it. I'll work on that next.
4. I had a feeling while making this that the spin dash was a bit of a crutch for poor level design. (I feel that the drill dive allows the player to interact with the terrain in a more interesting way) Maybe it still has a place.