For some weeks each time I reboot I see  systemd-journal-flush.service taking a long time to start, and "systemd-analize blame" confirms that:
         22.899s systemd-journal-flush.service

But, even taking so long, it starts properly at the end:
# systemctl status systemd-journal-flush.service
● systemd-journal-flush.service - Flush Journal to Persistent Storage
   Loaded: loaded (/usr/lib64/systemd/system/systemd-journal-flush.service; static; vendor preset: enabled)
   Active: active (exited) since sáb 2015-05-23 20:36:48 CEST; 10min ago
     Docs: man:systemd-journald.service(8)
           man:journald.conf(5)
  Process: 146 ExecStart=/usr/bin/journalctl --flush (code=exited, status=0/SUCCESS)
 Main PID: 146 (code=exited, status=0/SUCCESS)
   CGroup: /system.slice/systemd-journal-flush.service

may 23 20:36:25 belkin5 systemd[1]: Starting Flush Journal to Persistent Storage...

How could I know what is occurring to make it so slow?

My config is:
# cat /etc/systemd/journald.conf
#  This file is part of systemd.
#
#  systemd is free software; you can redistribute it and/or modify it
#  under the terms of the GNU Lesser General Public License as published by
#  the Free Software Foundation; either version 2.1 of the License, or
#  (at your option) any later version.
#
# Entries in this file show the compile time defaults.
# You can change settings by editing this file.
# Defaults can be restored by simply deleting this file.
#
# See journald.conf(5) for details.

[Journal]
#Storage=auto
#Compress=yes
#Seal=yes
#SplitMode=uid
#SyncIntervalSec=5m
#RateLimitInterval=30s
#RateLimitBurst=1000
#SystemMaxUse=
#SystemKeepFree=
#SystemMaxFileSize=
#RuntimeMaxUse=
#RuntimeKeepFree=
#RuntimeMaxFileSize=
#MaxRetentionSec=
#MaxFileSec=1month
#ForwardToSyslog=no
#ForwardToKMsg=no
ForwardToConsole=yes
#ForwardToWall=yes
TTYPath=/dev/tty12
#MaxLevelStore=debug
#MaxLevelSyslog=debug
#MaxLevelKMsg=notice
#MaxLevelConsole=info
#MaxLevelWall=emerg

Thanks