Hello Chris,

thanks again for your reply.

quoted

 5.10.0-0.bpo.3-amd64

It's probably OK. I'm not sure what upstream stable version this
translates into, but current stable are 5.10.27 and 5.11.11. There
have been multiple btrfs bug fixes since 5.10.0 was released.

I missed in your first email this line:

Ok, I am compiling 5.11.11.

quoted

[Mo Mär 29 09:29:21 2021] BTRFS info (device sdc2): turning on sync discard

Remove the discard mount option for this file system and see if that
fixes the problem. Run it for a week or two, or until you're certain
the problem is still happening (or certain it's gone). Some drives
just can't handle sync discards, they become really slow and hang,
just like you're reporting.

In fstab, this option is not set:
/dev/disk/by-label/DataPool1            /srv/dev-disk-by-label-DataPool1 
        btrfs   noatime,defaults,nofail 0 2

How do I deactivate discard then?
These drives are spinning disks. I thought that discard is only relevant 
for SSDs?

Regards,
Hendrik