On 15 Jul 2019, at 23.17, Ralf Hildebrandt via dovecot <dovecot@dovecot.org> wrote:

* Ralf Hildebrandt via dovecot <dovecot@dovecot.org>:
We're using dovecot (via LMTP) as a backup for all incoming mail.

I upgraded from 2.3.6 to 2.3.7 on the 12th:
2019-07-12 14:35:44 upgrade dovecot-imapd:amd64 2:2.3.6-2~bionic 2:2.3.7-8~bionic

and it seems that 2.3.7 is slower than 2.3.6 -- mail to the backup
IMAP box is suddenly taking quite some time to get delivered and is piling up
in the queue.

And alas, I had a look at the monitoring and found that disk IO has
increase A LOT after the update: https://www.arschkrebs.de/images/dovecot-disk-io-increase.png

I zoomed in and found that the sudden increace in IO coincides with
the update to 2.3.7 (14:35): https://www.arschkrebs.de/images/dovecot-io-2.png

Do the different disks have different kinds of data? Like you seem to be using external attachments and fts? Also your doveconf -n doesn't show what fts backend you're using. Or is it just half-configured and not really used?