Understanding delays when logging on the first time with a new user account

Depending on the configuration of your startup services, you may find that new users are unable to log on to a computer immediately (within the first 15 to 30 seconds) after a computer is rebooted.

By default, the Mac login window only requires the Disks and SecurityService startup services to start successfully to prompt for the user to log in. Authenticating users to Active Directory, however, requires the additional DirectoryServices startup service to be available. Starting the DirectoryServices startup service causes a 10 to 15 second delay before the LoginWindow can successfully authenticate new Active Directory users.