In the case of Whonix KVM 1 CPU per machine seems to cut it even for video playback. I pin 1 cpu per vm to avoid machines co-sharing a CPU cache - as extra hardening against sidechannel attacks on crypto libraries. I’d like to keep that if I can.
Overcommitting cpu will cause the host to lock up if the user runs more than 2 GW-WS pairs from experience.
Quadcore is pretty standard now.
Try playing around with the zram settings, compression algo choices to find a better trade off for CPU use vs memory savings. My other suggestion is to try zswap which might be more easy going on CPU since older memory pages can be swapped out unlike zram.