how to decide if acceptable to add vCPU to remedy CPU performance??

We have several XenApp 6.0 servers wherein end users often say, "The servers are slow."

The only thing that has been found is that CPU usage is noticeably high when this happens.

They already have 16 GB vRAM and 2 vCPU.

When and how can we determine if it's appropriate to add two more vCPU per XenApp server?? -- which would bring each XenApp server to 4 vCPU.

All the ESXi hosts each contain two quad-core processors.

All the XenApp servers are on Windows Server 2008 R2 SP1, which I understand allows us to add or remove vCPU without undue issues, please correct me if I am wrong.

Thank you, Tom