Hey, thanks for trying the demo and sorry for the somewhat late reply.
I've noted down these momentum issues, I'm pretty sure a system for this already exists and it's based on how high you fall from but with that being said, I'll take a look later since I know you get this slowdown when just jumping normally.
The Max Graphics Quality mode mostly just increases the render resolution but this shouldn't really be a problem on anything other than low-end PCs / laptops with integrated graphics.
Unfortunately there's no way for me to really know why you get slowdowns without more information like PC specifications and even then it might not help much.
I can only speculate this based on you being able to play monster hunter with that framerate:
It really shouldn't be a GPU issue, but you saying it only happens on max graphics indicates that it should technically be since 90% of what happens on max graphics vs high is GPU related.
It may or may not be a CPU issue depending on single core speed since the game is single threaded. (because GameMaker Studio is)
It could be that the fps fluctuates below the frame skipping threshold (I think it's 57 or 53 fps default) and as such turns on frame skipping for a few seconds and makes the fps go down to try and catch up, assuming the frame skip setting was set to dynamic.
So yeah, I can't really say for sure why you get FPS drops.
Anyway, thanks for the feedback, if you have any more information to give then feel free to do so. :)