There are power management settings that should be checked when running ESX on HP Proliant G6 and above or Dell PowerEdge 11th and 12th Generation servers. See VMware article for details: https://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1018206
The Proliant G8 that I examined having performance issues was set in the BIOS to use "HP Dynamic Power Savings mode" instead of "HP Static High Performance mode". This can have an impact on virtual machines ability to utilize the CPU of the host. This setting can be changed through iLO without the need to get into the BIOS directly to make the change. It does not require a reboot of the ESX host to change the setting this way, which is even better.