Cores presented to your machine by the hypervisor do not have a 1 to 1 relationship with physical cores. In the world of virtual machines, how many cores you have is meaningless.
For example, my VMWare server is a single 8 core processor, but my VM's only see 2 cores, as that's the way I prefer it and I believe VMWare recommends this or even one core. Those 2 virtual cores can access all 8 physical cores.
The only real way to find out your cpu performance is to run a benchmark.
If you had a host machine with 8 cores, and a virtual machine with 2 cores, the virtual machine will only possibly use 25% of the CPU resources of the host machine -- one vCPU cannot be schedule in parallel across more than one physical core.
However it is advisable to minimize the vCPUs in VMs because it greatly enhances scheduling -- if a VM has 8 vCPUs allocated to it, with some hypervisors that lack advanced co-scheduled it won't be scheduled until 8 cores are available, and in some situations it will sit on 8 cores during its duration whether it is only using 1 or all. If you have a large number of smaller VMs, scheduling can greatly improves.
This isn't true for all hypervisors and situations. You mileage may vary.
For example, my VMWare server is a single 8 core processor, but my VM's only see 2 cores, as that's the way I prefer it and I believe VMWare recommends this or even one core. Those 2 virtual cores can access all 8 physical cores.
The only real way to find out your cpu performance is to run a benchmark.