I experienced a very slow boot time (up to several minutes) for my Windows 10 VMs with Virtualbox on my Windows 10 host machine. As all tuning of the VM settings did not help, I finally solved the issue with the following command:
VBoxManage modifyvm <vmname> --largepages off
After that, the boot time was only a few seconds. You need to run that command for all your Windows 10 guest machines.