When you are logged in as Admin, navigating around the /admin section fires off several additional checks against the database. This additional load isn't very noticeable, but with several admin users at the same time, plus a bit of additional traffic from regular users, it could surely show up.
These extra queries happen on the /admin page itself, and check that the DB versions are up to date, etc. The Variables page may be loading the list of languages, perhaps. If you are not using the langcache, this is also a factor to consider.
The pages under /admin generally consume more RAM than usual - check earlier discussions about optimizing performance and minimizing memory footprint, specifically Apache/MySQL settings. The forum search is really good and if you search for maxclients, for instance, it'll lead you to the right posts
It is important to _not_ use admin accounts. If you have 3 users "administering" your website at the same time, you have way too many cooks in the kitchen. Things will break. Give them an 'everyday' unprivileged account, as well as their super-powerful accounts. Treat admin accounts as you treat the root account in Unix.