It types you need to install linux-image-amd64, but Whonix 14 use amd64 by default so I installed VirtualBox in my KVM.
When I go to Settings of VirtualBox VM System > Processor - the slider is grayed and I can’t use more than 1 processor and Acceleration tab is grayed I can’t access it.
There is only an IRC channel and maybe a mailing list. It is laggy because you are using an additional virtualization layer and nested virtualization is not really a priority for most vendors. I don’t think you will get acceptable performance any time soon. Though this might also depend on your workload, hardware and hypervisor. iirc from the benchmarks I saw a while back the the second layer was CPU wise mostly on a pentium level. Use your favorite search engine for more information, this is also not really a Whonix specific issue.