Thank you for your feedback!
Actually the mouse position is locked (if you click on the screen after starting; unfortunately it's not automatic on the web build) and I'm using the mouse movement as input.
I don't know why but sometimes it works well, sometimes it's stiff and delayed. I have no idea why, I guess it's related to how mouse input and physics updates interact in Unity(?)
P.S. your game looks very cool from the screenshots, I'd love to try it but I'm using Linux :(