Does shutdown idle VMs improve the performance?

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http://serverfault.com – Often our team members are coming to me with a compliant that their VMs are slow. Our team members suggested to shutdown some of the VMs temporarily and try to access the VM. But most cases that would not help. Assume that i have assigned 4 GB for and 2 CPUs for my VM. So ideally it should not face performance issue. As our ESXi 4.1 server has multiple VM in the same server (we have overcommited memory and CPU). Does shut down other VM really helps to improve performance or not? [Note : We are using ESXi 4.1 and our hardware is R710 server. We have more number of VMs in single server (HowTos)