This is not a "normal" performance issue, something is wrong somewhere.
Do you test Moodle courses as the site adminstrator? Can you reproduce this as a teacher?
Is your Apache configured properly?
You call the moodle site as http://some.dns.name I guess. Do you have the same thing in $CFG->wwwroot in your config.php?
Is your DNS properly configured?
Any chance you have local firewalls active?