Suggestions:
Some things to check: if there are any additional plugins installed? If there are any non-standard authentication methods enabled?
Basically check if either 1) there's an authentication plugin that's timing out, or 2) there's an enrolment plugin that's timing out. The authentication plugins are tried in the order in which they appear in the Manage authentication page. Some authentication plugins may try multiple servers, e.g. auth_ldap can try multiple servers, and one server may be unavailable but the connection attempt may need to time out before the next is tried.
On successful authentication any enabled enrolment plugins will enrol the user as needed. If one of these is attempting to connect to an unresponsive server, e.g. enrol_database, then this may have to time out before the login process completes.