If a Primary VM appears to be executing slowly, even though its host is lightly loaded and retains idle CPU time, check the host where the Secondary VM is running to see if it is heavily loaded.
When a Secondary VM resides on a host that is heavily loaded, the Secondary VM can affect the performance of the Primary VM.
A Secondary VM running on a host that is overcommitted (for example, with its CPU resources) might not get the same amount of resources as the Primary VM. When this occurs, the Primary VM must slow down to allow the Secondary VM to keep up, effectively reducing its execution speed to the slower speed of the Secondary VM.
If the Secondary VM is on an overcommitted host, you can move the VM to another location without resource contention problems. Or more specifically, do the following:
For FT networking contention, use vMotion technology to move the Secondary VM to a host with fewer FT VMs contending on the FT network. Verify that the quality of the storage access to the VM is not asymmetric.
For storage contention problems, turn FT off and on again. When you recreate the Secondary VM, change its datastore to a location with less resource contention and better performance potential.
To resolve a CPU resources problem, set an explicit CPU reservation for the Primary VM at an MHz value sufficient to run its workload at the desired performance level. This reservation is applied to both the Primary and Secondary VMs, ensuring that both VMs can execute at a specified rate. For guidance in setting this reservation, view the performance graphs of the virtual machine (before Fault Tolerance was enabled) to see how many CPU resources it used under normal conditions.