Most performance related problems with the ISA Firewall are due to DNS misconfiguration or disabling Path MTU Discovery. However, if you have a busy ISA Firewall in a large enterprise environment, authentication might be slowing down your ISA Firewall. If your ISA Firewall has to do with a lot of authentication requests, you can significantly speed up the Firewall by following the directions in this KB article:
http://support.microsoft.com/kb/326040/en-us
HTH,
Tom
Thomas W Shinder, M.D.
Site: www.isaserver.org
Blog: http://blogs.isaserver.org/shinder/
Book: http://tinyurl.com/3xqb7
Email: [email protected]
MVP — Microsoft Firewalls (ISA)