When starting my game with BattlEye enabled, I get constant stuttering/freezing whenever I press mouse/keyboard buttons.
This is typically caused by malware/adware/PUP constantly reloading into your game. Please try to scan your entire system with anti-malware software such as Malwarebytes Anti-Malware to remove it.
This answer is copied from BE's FAQ and slightly adapted to be Unturned-specific. Click here to visit the BE support page.