If it helps, I wanted to have a cold storage backup of this pool. So I plugged an USB drive to my Windows workstation, installed Resilio on it, and invited it to share these files too.
Suddenly, the transfer speeds went up to "normal" (40~80MB/s), and it appears the original server (SERVER) was transferring it to the desktop (cold backup), which was transferring them back to the second server (hot backup). But as soon as I pause the cold backup, transfer speeds go back to a halt.
And something new also happened: the sync manager on the SERVER started to crash - doesn't load. I then have to restart the service/jail, and it keeps working for a while before it crashes again.
I don't even know how to get started.