Try disabling all of the plugins, does it still load slowly?

If not, enable the plugins one by one and see if you can figure out which one is making it load slowly.

If it still does, try enabling MySQL's slow query log to see what might be causing that: Analyse slow-query-log using mysqldumpslow & pt-query-digest. Make sure you restart the MySQL service after editing /etc/mysql/my.cnf so the changes take effect:

sudo service mysql restart

After enabling it, submit a few posts and browse around the admin panel so that it collects some data, and then post the output of the following commands:

mysqldumpslow /var/log/mysql/mysql-slow.log
mysqldumpslow -a -s r -t 5 /var/log/mysql/mysql-slow.log

Finally, revert the changes you made to /etc/mysql/my.cnf and restart MySQL.