Why are all bulk mailings delayed by 5 hours?
I would look at the scheduled_date on the mailings in your database:

SELECT name,scheduled_date FROM civicrm_mailing;

If the scheduled_date is off, then it might be that MySQL has a time mismatch - or that specific users in Joomla have a different timezone setting than the default (not sure if this is possible in Joomla, but it can be done in WordPress/Drupal).

If the scheduled_date is correct, then I'd check to see that your cron user is running in the same time zone.  E.g. if cron is triggered by root and not the web server user, I'd expect this sort of issue to creep in.

EDIT: You mention that you're running cron via the "cli.php" tool.  That means that PHP is using a different php.ini file than when you do web-based PHP, and your timezone might be different.  If you have terminal access, check your timezone with php -i | grep zone.

Since you're using Siteground, I doubt they allow you to control the php.ini timezone.  Since that's the case, I would recommend using the HTTP Method of running cron, which will set the timezone based on the web-based timezone.