http://dnbible7sqskwppgaqohboxz7u5kzffjzdx3lxzpjtt3s6tzeiytb5ad.onion/bible/operating-systems/whonix/performance_tips/index.html
Here are the steps , you basically need to get into your BIOS / UEFI settings -> search for an option called something like Virtualization or VT-x -> enable it -> save settings and reboot. Then when you rebooted with the new settings, the “Processor(s)” slider should not be disabled any more.