After trying and trying with different solutions, installing truenas in USB and using SSD for slog, ARC2, etc the issue was much more simple. For unknown reasons the cache write of my hard disk were set to 0 in Truenas whereas in Gentoo and Debian I was enabled by default, not sure the reason of this.
The raid moved from 40mb/s, and dropping to 5 mb/s after few minutes, to achieve 250 mb/s without issues via SCP with Rsync (already copied 1 TBs of data and no issues)
Command:
hdparm -W 1 /dev/sdX
Thanks!