Poor performance experienced by your VMs may be related to processor power management implemented either by ESXi/ESX or by the server hardware.
One real world case I recently encountered with a customer involved VMware Horizon View and large delays experienced by their end users. Applications took unusually long times to open and general performance was quite bad. This was quite apparent when comparing the same applications on a thick client to running it on a virtual desktop.
After running through the usual checks consisting of VMware Health Analyzer, checking for over-subscription and over-utilisation there were no red herrings immediately apparent. What we did discover though (which is detailed in ‘Best Practices for Performance Tuning of Latency-Sensitive Workloads in vSphere VMs’) involved changing a BIOS setting on all of the ESXi hosts. Specifically the setting for power management on the ESXi HP Hosts to “Static High”, that is, no OS-controlled power management.
While we are working through the other recommendations provided in the VMware Health Analyzer report and have already made some changes to the configuration, nothing has resulted in a noticeable improvement with the exception of the power management setting. The customer has reported that after changing this particular power setting it has provided the most significant improvement in performance of anything previously attempted (hardware and software).