Hi,
The movement on slope is fine. I didn't have major issues with it. Perhaps a small issue is it kinda mess up with the jump button and can make the jump delayed.
With the current state of the game, I feel like you're aiming to make a high-speed 3D platformer, or a parkour game. It's good that you're focusing on the movement for now. I would suggest you also brush-up on the 3D animation since it will be the main draw of the game.
Another note I have for the game is that it really frustrates me when I fall down after getting the key. It dissolves after falling down and you have to start all over without any checkpoints. Since I know that this is still in early phase of development, I can bear the lack of polish for doing a bunch of retries . But I quit after around 15 tries. I just couldn't beat the first level. I think the slope at the very beginning is too high for you to go back. Here is the screenshot: I don't think the player can jump high enough to get over it. Or maybe you have to stand on the edge, but that requires too much precision for a level 1 challenge. Look at the time, it took more than 2 minutes to go get the key and get back. So having one or two checkpoints somewhere will be good.
Weirdly enough, all the UI have some polish quality to it, but the gameplay itself is very bare-bone. It looks like you're procrastinating on doing the hard more meaningful part of the game :p (I hope I don't sound too judgmental. It's just I often catch myself doing easier stuff instead of doing the more meaningful things because it's hard).
Keep working on it, and all the best!