Welcome to the community,
Two things you got to look at.
1. Change the virtual machine monitoring mode.
http://kb.vmware.com/kb/2012205
1. Change the HAL from multiprocessor to uniprocessor for single vCPU VMs, since it has 2 processor with 12 core, check if the HAL is set to multiprocessor. Also with just 1 vCPU your VM will give the expected performance, since the load on the server is going to be very less 4 to 5 users..
refer the below KB:
http://kb.vmware.com/kb/1077
http://support.microsoft.com/kb/234558
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