During the logon process, authentication happens at multiple stages; at minimum when a user logs on to the Web Interface and when the session-host server creates a session for launching the requested application. Citrix XenApp integrates with Active Directory. The authentication is therefore performed by a Domain Controller (DC) server of your domain.
Slowness in the Domain Controller response, caused by an overloaded server, can slow down the entire process. Worse, if the Domain Controller is unavailable, a domain member server may try to connect for 30 seconds before timing out and choosing a different DC.
Domain member servers choose the Domain Controller for authenticating users based on their membership to Active Directory Sites. If sites are not correctly configured or don't reflect the real topology of your network, a domain member server may decide to use a remote Domain Controller, through a slow WAN link, instead of using a Domain Controller on the same LAN...