slow performance external gateway Cognos 8.4.1 outside firewall

[maria]

Dear all,
We have a problem with performance in Cognos 8.4.1 on the Cognos portal run on the webserver with external gateway (win server 2003 ). The webserver is outside the firewall. Users log on to the website using External AD. The Internal and External AD are on the same forest.
The Cognos server is (Windows Server 2008 Standard 32 bit) is inside the firewall.
When running cognos portal internally (internal AD) then it's 2 seconds to run tabs and load pages. To run the cognos portal externally outside the firewall(external ad) then it's 30 seconds to load tabs and pages.

Any ideas?
Regards
Maria