.vhdx is not a supported format for VirtualBox vms. Change the option in disk2vhd to produce a .vhd image. You can then convert that to the native VirtualBox format .vdi . You may be offered the option to do that when you create the vm. I was last time I used disk2vhd to convert a physical machine to virtual .
Very slow operation suggests the Windows hypervisor is active. DO you see a green turtle with a V where you normally see just a white V on a blue background?
Turtle.PNG