FreeNAS iscsi degradation on replication?

Question

FreeBSD

Hello.
And so I use iscsi storage for VM hyper-v.
3 hyper-v nodes approximately 10 VMs.
Storage on freenas 9.10
Config:
CPU: Xeon 5620 (8 cores)
RAM: 32Gb (ECC)
raidz1 of 9 hdd WD 4000 GB + 128 GB L2ARC
there is ZVOL for 4 TB actually I announce it iscsi target
zvol was created with standard parameters:

and iscsi target :

This storage looks at hypervisors through lagg1 (LACP 2Gb)
+ backup-server is connected directly to it (storage) through lagg0 (LACP 2Gb)
In general, everything works, virtual machines are smart, they migrate quickly.
But if I start replicating this mount point to backup (I note that through a degenerate network)
then all the virtual machines just hang, replication runs at a speed of about 700Mbp, the iscsi speed degrades to almost 0, occasionally jumping up to 100Mbps.
As soon as replication ends, access to iscsi returns to normal and grows again to 1G
. At the same time, if at the same moment you copy the file from the server not via iscsi, but via cifs (essentially another mount point), then there are no problems and utilization up to 1G Why
is this behavior could be related?