Dedicate more CPU space to Virtual Machine

Hi,

I have an VM with 8 virtual cores dedicated to it. The physical VM has 8 cores and it's a Xeon CPU E5-2650.
However, when I am doing a stress test on the VM, the VM is getting really slow (normal of course) and it says CPU usage 100%.

But, when I check the physical CPU usage (hyper-V server 2012 core - Task Manager) it says well, maximum 4% usage.. and all my other VM's are still performing very good.

Is there a way to dedicate a much bigger part to a VM? I really need that option.

Thanks in advance!