There is nothing on the QuantaStor side for setting "async" on the NFS shares.
The Vmware best practice and performance guides don't say anything about "async" for NFS as they don't want people doing it.
Please review the link below:
OSNEXUS recommends using ISCSI for VMware datastores that require transnational or high performance workloads.
Using NFS in Vmware does use "sync" for all I/O transactions but iSCSI is able to do better client VM directed I/O for "sync" and "direct I/O" requests and VMware only forces sync operations for datastore metadata and locking operations.
Please review the link below for additional details: