Really slow IMAP performance

[Neil Jerram]

Please could you help me to understand and fix why my dovecot IMAP
performance is so bad?  I've read through a lot of the
performance-related material on the website, but I don't think that any
of it could account for slowness at the level that I am seeing.

The simplest scenario is moving a message from my Inbox to another IMAP
folder.  Using Gnus as the client, the whole UI freezes for about 2
minutes (which I assume is until the move is complete), and journalctl
on the dovecot server says:

Feb 24 10:24:24 arudy dovecot[1712]: imap(neil): Warning: Transaction log file /home/neil/dovecot-mail/.imap/INBOX/dovecot.index.log was locked for 98 seconds (Mailbox was synchronized)

or the same message with (rotating while syncing).

There must be something badly wrong in my setup, or perhaps in the spec
of the server that dovecot is running on.  What should I look at to
start understanding this better?

Many thanks - Neil