Very poor performance on iSCSI

Starting situation: Windows 2012R2 with MPIO with 2 gigabit adapters. RN316 with 6.9.3, 6 Drives full.

When a backup starts from the other NAS (same model) to this one (both DR or rsync) the iSCSI completely hangs.

I have replicated machines from another Hyper-v on this iSCSI and the replica stops, the disk is lost, then re-appears but writes at some kb/sec. Just trying to create a directory on the disk takes hours (literally).

So I started removing MPIO, using only 1 adapter. Same thing. Changed adpaters/cabling/switch: same thing.

I removed the backups and only kept the iSCSI for the Hyper-V replica, it works but with very low performance. I mean 300 mbit peak going up and down to 0. It takes forever to replicate VMs. I use a thick 2.5T volume (with and without sync writes, problems are the same)

I already discarded ReadyNAS solutions to have live hyper-v machines because it was badly off with performance, but I thought having replicas can be done. I was wrong.

Looking at the data I get from SNMP I discovered the NAS is completely overloaded on the CPU side. The attached graph is from the NAS receiving a single VM replica and doing nothing else.

The second graph is the network (peaks are at 300 mbit).

From the same server I can pump to another brand NAS near to full gigabit speed (same cisco switch, same network adapter).

What can I check to achieve decent performance and maybe have a backup starting without killing all the rest?

Thanks