Yeah I think the issue is in the core design of the game.
I needed a way to guarantee fixed time intervals between frames (due to the cloning mechanic of the game), so I do everything inside FixedUpdate. This is very much not advised usually, and so I think it causes what feels like a stuttery framerate.
Unfortunately I didn't notice how bad it was in fullscreen until towards the very end of the game jam, but hopefully its not too bad as to stop people from being able to play.
Thanks for playing!