I don't think Joomla would have that slowdown on Moodle - they should be separate databases, so they don't impact on each other. I've just tried to open the page, and the apart from a high load average of 3.75, there seems to be the slowdown that has been experienced before on Moodle 1.7.
So, it might be worthwhile going to Samuli's suggestions and resetting the theme back to the default standardwhite. If there is still no change, then an upgrade to 1.8.3+ would be worth trying also. Make sure you've gone through the suggestions in the Performance FAQ to help improve your server , and be careful to do a difference check on your database files when you upgrade.