What is your question? You haven't even used half of your CPU resources (wasting them).

50% is little usage. You still have 50% unused CPU resources. You could route much more traffic.

Have You verified the core allocation, on the virtual host?

Take KVM as an example:

I can allocate (say) 4 cores, from my single CPU. But, then, I can tell the virtual host to allocate 8 cores, to the virtual machine. It is not a good idea, but can be done.

So, question: did you really allocate 4 real cores to the virtual machine? Or, by mistake, is it using only one hardware core? It would explain the CPU usage you are seeing: since there is only one hardware core to do the work, it would look more loaded, when you carve it in four.

Winbox shows 1 CPU, so only one core configured in your VM settings.
What RouterOS config you have? Firewall, NAT, any other config? In any case, 50% CPU usage is normal.

2x Intel Xeon E5-2680v2
20Core / 40thread

2x Intel Xeon E5-2680v2
20Core / 40thread

Well, it still doesn't answer the question. You have 20 cores, 40 threads, to use. When creating the virtual machine did You allocate 4 cores? To be perfectly clear: it is possible to allocate more threads than cores - you can allocate one core and 4 threads to the machine. It would show up as a machine with 4 cores (or CPUs, by your configuration) - but will have only one hardware core available to work these 4 threads.

The expected result is something on the same lines you are reporting.

P.S. - Last time I checked, the official recommendation from Intel was to disable HyperThreading on virtual hosts.