Several people have asked me in the last few weeks about problems with the OWA form coming up too slowly. In many cases this might be related to not publishing the CRL for the site and the Web browser is configured for CRL checking. However, something I've taken for granted is that ISA 2004 Firewall users have updated their ISA Firewall to Service Pack 2.
If you haven't upgraded your ISA Firewall to Service Pack 2, then you should do that now. I've heard some "tribal knowledge", especially in the SBS space, that you shouldn't install ISA SP2 because of some non-specific problems. While the ISA Firewall does introduce a new CARP algorithm that can play havoc with your Direct Access configuration, this is easy to fix. Check out Stefaan Pouseele's article on this subject at http://blogs.isaserver.org/pouseele/2006/07/
Another reason to upgrade to SP2 is that it will fix your OWA performance issues. There was a memory leak in pre-SP2 where there was a leak of 8 handles for each successful FBA log on. That leak goes away with SP2.
For more information on this problem, check out the KB article related to this issue at: http://support.microsoft.com/kb/897717/en-us