ESX & ESXi Performance

Absolute best tool for ESX/ESXi Performance .Best way to see the performance of 
VM on a Host is using .This gives you real-time resource utilization statistics

esxtop for ESX
resxtop for ESXi  using VMA

Q- Quit
C- Switch to CPU Utlization Screen
1)look for CPU Ready value this is the amount of time it waits for Disk  or Network.
2)The %CSTP value represents The %CSTP value represents the amount of time a virtual machine with multiple virtual CPUs is waiting to be scheduled on multiple cores on the physical host. The higher the value, the longer it waits and the worse its performance.  Lowering the number of vCPUs reduces the scheduling wait time. the amount of time a virtual machine with multiple virtual CPUs is waiting to be scheduled on multiple cores on the physical host. The higher the value, the longer it waits and the worse its performance.  Lowering the number of vCPUs reduces the scheduling wait time.

M- Switch to memory Utilization Screen
D- Switch to Stroage (disk) adapter resource Utilization Screen
U- Switch to Stroage (disk) device resource Utilization Screen
V-Switch to Stroage (disk) VM resource Utilization Screen
N-Switch to Network resource Utilization Screen
I-Switch to interrupt Utilization Screen
P-Switch to Power Utilization Screen

These options apply both for ESX and ESXi
http://communities.vmware.com/docs/DOC-9279