I run this combination on my AMD machine( VirtualBox and W10). I would try telling Win 10 it has fewer resources to work with such as 2 cores and 3 GB of ram.
On my AMD rig I just enable the VM multicore function in the MB BIOS and VirtualBox sees the service and runs multicore VMs.
One thing I've noticed W10 is borrowing from W8 and showing my e taskbar like its ready but it's still loading for awhile.
My main bottleneck seems to be the Virtual HD it's only SATA. No SATA 2 or 3 available in the free version.