EDA500 on RN516 - Scrub very slow

Interresting that there is no response form Netgear on this one.

An update:  I stopped the scrub and re-scheduled the start.  It did re-start (from scratch, not where left off) and was going a little faster -- it was going to take days instead of weeks.  Once again, I lost access to shares and admin interface, but SSH worked this time.  cat showed fwbroker processes taking up a huge chunk of the CPU.  I stopped the sync via SSH (btrfs scrub cancel) to see I could then access my data.  Sure enough, the fwbroker tasks stopped or settled to near zero CPU and share and admin access were restored.  I then resumed the scrub (btrfs scrub resume) and it is going MUCH faster, though share and admin access are still badly affected.  It had taken 56 hours to scrub a little over 2Tib and in the last 12 hours has done another 4 (now only visible via SSH (btrfs scrub status) since the GUI is not aware of the scrub restart.

• Why does scrub on the EDA500 take so much longer than the main array?
• Why does scrub on the EDA500 take up so much CPU time that the NAS becomes inaccessible, yet it's not helping the scrub duration?  That never happened on the array of my older Pro's running 6.x or the main array of my RN516.
• Why can't we resume a scrub via the GUI?
• Why does the resumed (GUI not in the loop) scrub work so much faster (times more like the main RN516 array)?