Citrix sets a registry key to 5000 by default if installed on a machine with quad core processors, 10000 on dual core processors, and 20000 on a single core processor.
A significant performance issue occurs when Citrix is installed with quad core and the processors are later lowered to dual or single core. Initially, Citrix install sets this reg key to 5000 and once the cores are lowered to (say to dual core), the Citrix program does not automatically update this registry key.
This registry key is used to partition out 5,10, or 20% of the processor to be used by Citrix. Citrix can become significantly slow if this registry setting does not coincide with the core processors. This is the case if the number of processors have been lowered on a VM after Citrix Presentation server has been installed. You have to changed the value in this registry key to reflect the number of cores in place on your servers and then restart the Citrix CPU Utilization Mgmt/Resource Mgmt Service.
Reg key : HKLM\Software\Citrix\CTXCPU\policy
Edit the line : NT AUTHORITY\SYSTEM,cpu.reservation=xxxxx
Where xxxxx represents the number of cores on your server.
5000= 4 quad core
10000 = dual core
20000 = single core