Possible issue with software RAID 1 in case of disks with different speed

Hello,

I am using Debian stable for years (currently, release 13; before, 12,
11, 10, etc. starting with 3.0, if my long-time backups are complete).

For some years now, I am using software RAID with two RAID 1 sets with
three 8TB disks each, one for my production data (/home, /var/lib,
/var/www, etc.) and the second one for a nightly backup of my
production data (three disks each because HDDs failed quite frequently
then, and recovery was slow).

Initially, I used external disks connected mainly via USB 3.x, and also
via USB 2.0, when I ran out of USB ports. Reason for USB was that I
couldn't/wouldn't afford a server with slots for accomodating six 3.5"
HDDs.

After migrating the failed 3.5" HDDs to 2.5" SSDs one-by-one over the
years, I finally migrated five disks into a rack-mount server and
connected the sixth disk via USB 3.x (I couldn't migrate all six disks
because one slot was still occupied by the boot disk).

I run nightly rsync jobs ('managed' by Back In Time) to backup
everything from /dev/md0 (boot) to /dev/md1 (production) and then to
/dev/md2 (backup).

Now, as long as the sixth disk was still connected via USB 3.x, the
rsync job and a kworker job were 'blocked' after some hours of rsync-
ing, and the console displayed some 'sync' errors, and I had to press
Ctrl+Alt+Del to reboot the system because login didn't work anymore.

I flagged the USB 3.x disk 'write-mostly' and 'nofailfast' but this
didn't resolve the issue.

Only after I added two NVMe SSDs as boot disks and migrated the sixth
SSD into the sixth slot, everything runs fine.

Conclusion:
I suspect software RAID 1 has issues if one disk of a three disk RAID 1
set is significantly slower than the other two disks.

Everything works fine for me now, but in case ...

Kind regards, Christian