Some at least don't even bother to send relative mouse movements, what matters is where you click. So they disable the guest mouse-pointer and rely on the host

Some at least don't even bother to send relative mouse movements, what matters is where you click. So they disable the guest mouse-pointer and rely on the host instead and then only send positions every now and then and when you are actually doing something interactive. Synergy types of applications don't have that freedom because the host mouse cursor don't extend to the other devices display.