request optimization

Try changing your return type to the root object for VMs, VMWVirtualMachine,

and the path on root reference to VMWModel/virtualCenters/virtualMachineCollection/virtualMachines

Your where-clause path would then be to a property on or below the return object. 

There is no purpose to making the return object anything below VMWVirtualMachine, unless you are creating 1 row per object in a 1-Many relationship with the root object, such as processors or logical disks.

In this case, the path in the where clause comparison is triggering additional unnecessary walking back up the tree from the return object, which can work but not very efficiently in large environments.

Note that the aggregations are going to cause performance issues with large data sets, and how they are created can dramatically change performance.  Always test performance first with no filter or aggregation, then add filters, then add aggregations.

Aggregations are generally limited in capabilities in Queries, anyway.  In general, I would remove the aggregation, do the filter here, then feed the output to a WCF function to do the aggregation(s) as needed.