Solved!
I've found the problem:
I think there is a config issue on default Intel QNAP TCP config (factory default):
Default Receive TCP Window on Intel QNAP = net.ipv4.tcp_rmem = "4096 1638400 1638400"
Default Receive TCP Window on ARM QNAP = net.ipv4.tcp_rmem="32768 87380 16777216"
I think Intel's config had a too low minimal window
As soon as I changed the value on the Intel to the values on the ARM, ssh data transfer went double and beyond
So I tweaked the TCP Windows (send and receive), and got my beloved 1Gbps per job
code (ssh root on QNAP )
sysctl -w net.ipv4.tcp_rmem="131072 524288 16777216"
sysctl -w net.ipv4.tcp_wmem="131072 524288 16777216"
Also, I think it's active on default, but not a bad idea forcing autotuning:
sysctl -w net.ipv4.tcp_window_scaling=1
I hope it's useful If anyone has the same issue.
Just be sure to include the tweaks on autorun.sh to survive reboots
Thanks