I was doing some I/O testing between iSCSI and SMB/CIFS for a Windows server and I was quite surprised that SMB/CIFS *DRAMATICALLY* outperformed iSCSI; my experience is that iSCSI is better if not comparable to SMB/CIFS, so I'm wondering if I just have some configuration issue with OMV. I doubt that it's on the client side or network, as an iSCSI connection to a different iSCSI target from the client produces acceptable performance.
There are a lot of parameters to set on the target that I don't understand the implications of (mostly). I just used the default values (but did check "Immediate data" based on it's suggest to achieve better performance)