EDA500 on RN516 - Scrub very slow

Can you be more specific as to what it is doing if not just a BTRFS-level volume scrub?  btrfs scrub status /eda1 returned data consistent with the progress bar in the GUI when I initiated the scrub via the GUI.  In fact, the scrub I started with SSH also shows up in the GUI, progressing at the much accelerated pace.  It's going a tad slower than the main volume -- I estimate it'll take about 18 to 20 hours -- but that's acceptable.  The speed of the GUI-initiated scrub is not.

And even if it's doing something more, why does the scrub of my 14.5TB RDA500 volume take 54 days compared to 18 hours for my 27.3TB main volume?  I assume that scrubbing the main volume is also doing that something more.  Scrubbing the main volume uses about 50% of the CPU time -- enough to slow other processes some.  Scrubbing the EDA500 via the GUI takes almost none.  Scrubbing via SSH command takes about the same amount as the main volume scrub.  Whatever causes that lack of CPU usage certainly seems like the probable root cause to me.