[Dovecot] dsync incredibly slow

Hi!

While performing a dsync from cyrus imap to dovecot 2.2.12, dsync seems to stop for perhaps a minute without disk / cpu / memory activity (that I can see). This happens several times per sync per user so it takes an enormous amount of time to sync just a couple of gigs of mail.

dsync -D -v -o mail_fsync=never mirror -f -R -u user@example.com imapc:

What could possibly be the cause of this slowness? Is it protocol dependent or just dsync it's self?

Thanks!