From what I can tell is that the game is Frame rate related - speed jump gravity everything , and if your have a lower end computer even on the lowest settings you are going to be slow and some cases looking down at the ground is the only thing you can do to increase fps , I also thing the View Distance is too high even on low maybe ad a fake fog of sort
Its confusing how you say you play tested this and did not consider lower hardware running this lol
Don't get mad at me juts helping out for future project you make