periodic daily takes a very long time to run (14-stable)

Hello list,

I'm asking this in order to determine whether I've got something misconfigured or
maybe my expectations need recalibrating, or maybe both.

context is rpi4b with 8GB and usb3-connected 1tb zfs hard disk.
/etc/daily.local has been moved out of the way to try to fix the issue.

When periodic daily runs, it seems to take a long time. It's running now and has
been running for overan hour so far.

top shows 

   PID   USERNAME    THR  PRI   NICE SIZE RES   STATE    C   TIME    WCPU COMMAND
   21649 root          1  20    0    18M  6668K zio->i   1   0:40   0.53% find

There's plenty of resources

93 processes:  1 running, 92 sleeping
CPU:  0.1% user,  0.0% nice,  0.5% system,  0.0% interrupt, 99.4% idle
Mem: 202M Active, 688M Inact, 69M Laundry, 3079M Wired, 216K Buf, 3780M Free
ARC: 975M Total, 552M MFU, 244M MRU, 808K Anon, 21M Header, 158M Other
      229M Compressed, 905M Uncompressed, 3.95:1 Ratio
      Swap: 12G Total, 12G Free

but the 'find' thing does heavy disk i/o and everything else reading from or writing 
to disk slows to a crawl.

# cat /etc/periodic.conf
daily_status_smart_devices="da0"
daily_status_include_submit_mailq="NO"
daily_clean_hoststat_enable="NO"
daily_queuerun_enable="NO"
daily_submit_queuerun="NO"
daily_scrub_zfs_enable="YES"
daily_scrub_zfs_pools=""
daily_scrub_zfs_default_threshold="7"
daily_status_ntpd_enable="YES"
daily_clean_disks_enable="YES"

zpool last scrubbed on the 21st; it's not happening right now

there's the following in /etc/sysctl.conf relevant to the context:

vfs.zfs.min_auto_ashift=12
#
# filesystem
vm.pageout_oom_seq=120
vm.pfault_oom_attempts=-1
vm.pageout_update_period=0
# vfs.zfs.resilver_min_time_ms=5000
vfs.zfs.arc.min=536870912
vfs.zfs.arc_max=1073741824

The OOM settings are there for poudriere.

# zdb | grep ashift
             ashift: 12            

# ps xx | grep periodic
68824 11  S+      0:00.00 grep periodic
16115 14  I+      0:00.00 /bin/sh - /usr/sbin/periodic daily
17425 14  I+      0:00.00 lockf -s -t 0 /var/run/periodic.daily.lock /bin/sh /usr/sbin/periodic LOCKED daily
17747 14  I+      0:00.01 /bin/sh /usr/sbin/periodic LOCKED daily  <=== why are there two of these?
19460 14  I+      0:00.00 /bin/sh /usr/sbin/periodic LOCKED daily  <===
19921 14  I+      0:00.00 /bin/sh /etc/periodic/daily/100.clean-disks <=== also here
21312 14  I+      0:00.00 /bin/sh /etc/periodic/daily/100.clean-disks <===

I'm not sure if it's relevant, but the disk, when the zroot/zfs was initialised, was set
to enforce 4k blocks, geli-encrypted swap, geli-encrypted fs. Because it's an rpi4, there's
no aes-ni equivalent hardware. But the slowdown doesn't show much in other areas, only a couple,
like this problem and when git updates src or ports. smartctl shows no remapped or pending blocks.

What else can I do to debug the issue?
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