As per version 188.8.131.52.6-developers-only, nested virtualization in Whonix-Host (KVM) is extremely slow.
Meaning booting Whonix VMs in Whonix-Host takes around 3 to 5 minutes. VMs are extremely slow to the point of not being usable at all.
Tested on a i7 processor (4 cores allocated to Whonix-Host) with more than enough RAM.
Nested virtualization is not expected to be fast anyway, but it used to be fast enough for testing purposes.
See https://phabricator.whonix.org/T914 for the related discussion.
@Patrick please advise on how to add the relevant tags here. Couldn’t find the option.