Microsoft Knowledge Base Article 916474 contains the following introduction:

You have created an external trust between two Microsoft Windows Server 2003 Service Pack 1 (SP1)-based forests by using fully qualified domain names (FQDN). However, in this case, pass-through authentication of the user accounts in the domain that did not originate the trust (the TARGET domain) may take lots of time. This article discusses how to optimize pass-through authentication after you create an external trust between two Windows Server 2003 SP1-based forests.