iSCSI best practice and performance

[Razorblade]

FTP transfer blocking iSCSI

Hello,

my Freenas box serves as FTP server and iSCSI target for an ESXi host.
I have enabled MPIO for iSCSI.

The datastore for my FTP is on one RAIDZ volume and my iSCSI datastore is on one mirrored volume.
FTP traffic is handled via my onboard NIC and iSCSI traffic is going through two separate PCIe NICs in separate subnets. So those networks are physically separated from each other.

Since I enabled MPIO on my Freenas box I have horrible transfer speeds on my iSCSI network while copying files to my Freenas box via FTP. Practically iSCSI stalls while data is being transferred via FTP. And so my VMs become unresponsive. Windows Resource Monitor is showing very high Disk queue lengths while an FTP transfer is going on.
Before I enabled MPIO this was not an issue, I was able to up/download files via FTP with 100MB/s and working with my VMs at the same time.

What can I do to restore the original performance?

If you need any more information, please let me know.

Thanks in advance