> First of all, thank you for your time!
No problem. If the information was useful, rate the post as "Useful".
> So, you basically mean that an other website from an other server uses the computer, where my page is, to store pictures, documents under an other account, next to mine?
Yes, that is what shared hosting means. See http://en.wikipedia.org/wiki/Shared_hosting
. You have to go through the description of your hosting plan to be sure.
> If yes, that means, that I can't really do anything about it. Can I?
Yes, pay more and move on to a better hosting plan.
> I also recognised that the performance of my site varries from different times of the day. Yesterday evening for example it took about 20 seconds to load a single page today is a little better.
The server is over-booked.
> Unfortunately, the last time I've used Linux was about 15 years ago (an even than I could do only the most basic things) so I can't say that I have enough expertise to do a proper analysis and tweaking.
I don't think a proper analysis is worth the trouble. See other discussions related to GoDaddy in moodle.org. You can use the "'Using Moodle' advanced search facility" mentioned in the introduction to find those posts. My feeling after seeing so many negative reports is, why people still go them? Because of their advertising. I have the different grudge with them: http://moodle.org/mod/forum/discuss.php?d=193106