Haha I am very sure it is not your coding - I had the exact same issue with one of my previous Jams, the way it responds is just WebGL converting the mouse I can tell. I had to add a sensitivity slider in my one too and you may find you still have to have that slider really really low, it's a weird webGL quirk.
I often ponder whether to have a download rather than browser because it changes the input sensitivty so much, I bet it plays very differently in editor/executable.
You did a great job!