http://dnbible7sqskwppgaqohboxz7u5kzffjzdx3lxzpjtt3s6tzeiytb5ad.onion/bible/operating-systems/whonix/performance_tips/index.html
If the “Processor(s)” slider is not disabled, set it to the middle value (i.e. if the maximum is 4 CPUs set it to 2 or if the maximum is 8 CPUs set it to 4). If you can not move the slider you only need to do one additional step, which is enabling an option called “VT-x technology” in your BIOS or UEFI settings.