@Bee_rii Thanks for the updated and glad you got it sorted.
It would be good if Rockstor’s network page reported the sync speed achieved by at least the local port. That might help to diagnose such instances. At least if they were local to the plugged in cable. E.g via ethertool:
ethtool p4p2
or whatever the device is called.
I recently myself spent ages tracking down, and initially working around, a long DHCP delay only to find I’d inadvertently used a single switch port previously configured as part of a Link Aggregation Group (LAG) via Link Aggregation Control Protocol (LACP). See the following forum post where I tested this setup in Rockstor:
Switched to a regular port and, as if by magic, the delay returned to normal. I’ll not go into the initial work around as it was embarrassingly deep for the sake of using the wrong port. @Flox I’d meant to update you on this finding by the way re our PM on this recent issue 
. Sometimes, (read mostly), it’s fairly easy to over think stuff. Especially when our technology is so deep these days.
Still your should be able to near saturate that ethernet link. You could try the iperf trick to double check you have a descent lower level connection, just in case.
Hope that helps.