I have a NAS which provides SMB and iSCSI targets, and a server which is directly connected via a 10 GbE link (no switch in between).
I experience slow speeds with the iSCSI target in contrast to "high" (expected) speeds with a SMB "target".
See the speed comparison between SMB (Y:) and iSCSI (E:):
How to investigate the performance issues with the iSCSI target?
NAS: Synology RackStation RS3617xs+