very slow mouse in virtualbox. how to fix it?

You have to make sure that you install Guest additions inside your system, it will help with mouse movement. As well as make sure that you enable virtualization in Bios. It exists under CPU features with different names like

Intel Virtualization, Virtualization or vt_x

Make sure mouse integration is enabled. If you have two mice pointers it’s not enabled. Clicking the mouse icon on settings tab fixed it for me.