replication delays

[djk]

I have two dovecot instances that replicate to each other. Mail arrives 
into "post" and it is generally accessed via IMAP on "france". Changes 
(moves/expunge etc) on "france" seem to replicate back to "post" more or 
less immediately. But this seems to happen only for this user. (Some) 
other users' (e.g. me) mail generally replicates more or less 
immediately in both directions.

Nov  5 17:40:55 post dovecot: lmtp(willem at xxx): xF9mK6eUO1YsFQAAdYyp2A: 
sieve: msgid=<20151105173923.14970.95605 at ssa-app-xxx>: stored mail into 
mailbox 'SSA'
Nov  5 17:41:00 post dovecot: lmtp(willem at xxx): yV9mK6eUO1YsFQAAdYyp2A: 
sieve: msgid=<20151105173924.14970.91073 at ssa-app-xxx>: stored mail into 
mailbox 'SSA'

Nov  5 20:39:09 france dovecot: dsync-server(willem): save: box=SSA, 
uid=3130, msgid=<20151105173923.14970.95605 at ssa-app-xxx>, size=9724
Nov  5 20:39:09 france dovecot: dsync-server(willem): save: box=SSA, 
uid=3131, msgid=<20151105173924.14970.91073 at ssa-app-xxx>, size=9197

Any suggestions as to where to start looking? As you can see I am using 
2.2.18 with mdbox and a simple all vmail based system with not many 
users. I am not seeing any error messages in my mail logs that suggest 
there any delays happening. Although I have seen a few things like this 
happening from time to time a busier account:

Nov  5 12:12:07 france dovecot: dsync-server(djk): Error: Couldn't lock 
/home/imap/djk/.dovecot-sync.lock: Timed out after 30 seconds
Nov  5 12:12:13 france dovecot: dsync-server(djk at tobit.co.uk): Error: 
Couldn't lock /home/imap/djk/.dovecot-sync.lock: Timed out after 30 seconds
Nov  5 12:16:49 france dovecot: dsync-server(djk): Error: Couldn't lock 
/home/imap/djk/.dovecot-sync.lock: Timed out after 30 seconds


root at france:/var/log# dovecot -n
# 2.2.18: /etc/dovecot/dovecot.conf
# Pigeonhole version 0.4.8 (0c4ae064f307+)
# OS: Linux 3.13.0-66-generic x86_64 Ubuntu 14.04.3 LTS
auth_verbose = yes
doveadm_password = <some secret string or other>
first_valid_uid = 2000
imapc_features = rfc822.size fetch-headers
imapc_host = <imap hostname>
imapc_port = 993
imapc_ssl = imaps
imapc_ssl_verify = no
imapc_user = %n
lda_mailbox_autocreate = yes
login_greeting = IMAP ready.
mail_attachment_dir = %h/attachments
mail_attachment_min_size = 64 k
mail_location = mdbox:%h:INBOX=%h/mail
mail_plugins = " fts fts_lucene mail_log notify notify replication"
mail_prefetch_count = 20
mailbox_list_index = yes
managesieve_notify_capability = mailto
managesieve_sieve_capability = fileinto reject envelope 
encoded-character vacation subaddress comparator-i;ascii-numeric 
relational regex imap4flags copy include variables body enotify 
environment mailbox date index ihave duplicate
mdbox_rotate_size = 10 M
namespace inbox {
   inbox = yes
   location =
   mailbox Drafts {
     special_use = \Drafts
   }
   mailbox Junk {
     special_use = \Junk
   }
   mailbox Sent {
     special_use = \Sent
   }
   mailbox "Sent Messages" {
     special_use = \Sent
   }
   mailbox Trash {
     special_use = \Trash
   }
   prefix =
}
passdb {
   args = /etc/dovecot/dovecot-sql.conf.ext
   driver = sql
}
plugin {
   fts = lucene
   fts_lucene = whitespace_chars=@.
   mail_log_events = delete undelete expunge copy save mailbox_delete 
mailbox_rename mailbox_create
   mail_log_fields = uid box msgid size
   mail_replica = tcps:post:<same port as doveadm port>
   sieve = file:~/sieve;active=~/.dovecot.sieve
}
postmaster_address = postmaster at tobit.co.uk
protocols = " imap lmtp sieve sieve"
service aggregator {
   fifo_listener replication-notify-fifo {
     mode = 0666
     user = vmail
   }
   unix_listener replication-notify {
     mode = 0666
     user = vmail
   }
}
service auth {
   unix_listener exim-auth {
     group = Debian-exim
     mode = 0660
     user = Debian-exim
   }
}
service config {
   unix_listener config {
     mode = 0666
     user = vmail
   }
}
service doveadm {
   inet_listener {
     port = <some port or other>
     ssl = yes
   }
}
service imap-login {
   inet_listener imap {
     port = 143
   }
   inet_listener imaps {
     port = 993
     ssl = yes
   }
}
service lmtp {
   inet_listener lmtp {
     address = 127.0.0.1
     port = <another port>
   }
   unix_listener lmtp {
     mode = 0666
   }
   user = vmail
}
service managesieve-login {
   inet_listener sieve {
     port = <sieve port>
   }
   process_min_avail = 0
   service_count = 1
   vsz_limit = 64 M
}
service managesieve {
   process_limit = 100
}
service replicator {
   process_min_avail = 1
   unix_listener replicator-doveadm {
     mode = 0600
     user = vmail
   }
}
ssl_cert = </etc/dovecot/tobit.crt
ssl_client_ca_dir = /etc/ssl/certs
ssl_key = </etc/dovecot/private/tobit.key
ssl_protocols = !SSLv3 !SSLv2
userdb {
   args = /etc/dovecot/dovecot-sql.conf.ext
   driver = sql
}
protocol lmtp {
   mail_plugins = sieve
}