It's the way the jump seems to not react at all and feels way too smooth. As for screen response, you may want to look at using Lerp on the camera movement, Brackeys has a great tutorial on it. One way I could think of to tweak the jump would be to make it a little bit higher, and to make the jump height increase a little bit when the button is held like it does in the mario games and many other platformers. Best of luck!
Thank you for your feedback TheExistingOne, and sorry for the late response. I have tried using Lerp, but it simply didn't work as I wanted to, even after hours of trying to make it work. I will have to look at that in the future. Also, the jump height does depend on how long you hold the button, it's just that the jump isn't very high, so it's probably not noticeable. If you find the diamond, which makes you jump higher, it should become more noticeable.
Regardless, I understand your frustrations with the game, and am very thankful for your feedback.