Joomla site extremely slow

[pmeenan]

Yeah, if he is trusted then it should be fine. I don't have enough experience with Joomla to know what will happen if you swap templates but it should be fine (usually it just impacts how the site looks and doesn't materially impact the database).

SSH is basically the same thing as if you were sitting in front of the server logged into it. You can do just about everything you would be able to do (as long as your account has permissions to do it). A sudo (or root) account would let you do anything you wanted - install software, change the OS configuration, reboot, etc.