MSDP manual queue processing delay

Strange issue on a replication-target appliance (5230 - v.3.1):

MSDP fills up, I expire some images, and initiate a manual queue processing run.  I'm watching the storaged.log and it has been over an HOUR since I submitted the queue processing command:

June 21 14:03:22 INFO [139655922538240]: Queue processing finish.
June 21 15:06:28 INFO [139655922538240]: Queue processing triggered by external request.

This is an awfully long delay - so I checked the last couple "regularly scheduled" queue runs:

June 21 00:20:03 INFO [139655922538240]: Queue processing triggered by external request.
June 21 02:07:28 INFO [139655922538240]: Get 68944-69076 tlog files to be processed from /msdp/data/dp1/pdvol/queue

June 21 12:20:03 INFO [139655922538240]: Queue processing triggered by external request.
June 21 14:03:11 INFO [139655922538240]: Get 69077-69104 tlog files to be processed from /msdp/data/dp1/pdvol/queue

Anyone else seen this?  System is not busy according to top:

top - 16:20:39 up 13 days, 22:53, 2 users, load average: 1.27, 1.20, 1.30
Tasks: 724 total, 1 running, 723 sleeping, 0 stopped, 0 zombie
%Cpu(s): 0.3 us, 0.2 sy, 0.0 ni, 96.7 id, 2.7 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem : 13166478+total, 3368640 free, 11015217+used, 18143968 buff/cache
KiB Swap: 68272128 total, 56560720 free, 11711404 used. 13822336 avail Mem