I don't know what you mean about the speed. If you mean the momentum, it's normal, there is friction but the engine keeps your speed.
The single slow jump could be terrible and make the game feel like you are under water.
So I won't do it.
For your point 3, the game is not finished, but adding levels isn't a priority since I'm busy working on my engine.
I understand, about speed is much more about the high speed gain itself, for example, in the Sonic of Genesis, using the trees behind in Green Hill as base, starting from the first if it reaches the maximum speed in the seventh, however in the project the gain is very high. Thinking about it might be the scenarios that are very small, it would be possible to put some areas for testing in the future.
On the leap I do not know, using Sonic 2D as an example, Sonic's jump is about 3 Sonics above him, I'm thinking on the side of breaking the game itself, jumping from one side to the other of the map and finishing the phase in 20 seconds seems a bit sad.
On performance the game has its downfalls, but it seems that in the next update you will lock the frame rate at 30fps, maybe even improve the rendering of the game.
Returning to talk about the jump, Sonic X-Treme was going to have phases differentiated for each character, but I do not know, maybe it's better to create a single phase with different paths for each character, maybe save time.