Recently moved a VirtualBox Win10 VM to a Windows 10 host.
Originally the VM had been created on a Linux host and ran fine there.
On Windows 10 it was running very slow.
After some debugging, looking for problems with BIOS settings or video card support I found that the Avast Antivirus installed on the Win10 host was interfering with Virtualbox.
Links:
https://www.reddit.com/r/avast/comments/6hrlpv/avast_slowing_down_virtual_machines/
https://forum.avast.com/index.php?topic=162445.0
So, on your Windows host computer in Avast Free Antivirus
[(19.4.2374 (build 19.4.4318.460) in my case]
under Menu > Settings > Troubleshooting
untick/de-select 'Enable hardware-assisted virtualization'
and reboot
Then your VM should run at the correct speed.
Note: There is a warning for that setting saying you might be at increased risk of a virus infection from your VM. Turn off at your own risk. Or use a different Antivirus product.
I have not tried if VMWare or Hyper-V products are affected in the same way.
