Try specifying a domain controller to talk to in "Advanced Settings" to see if it speeds up login.
Here is some information from the MCU Admin Guide:
By default, the management console contacts Active Directory through any site, domain, domain controller, or global catalog that is available. To limit how the console contacts Active Directory, click Advanced Settings and specify which sites, domains, domain controllers, or global catalogs you want the console to contact.
To configure advanced Active Directory settings
1. Log into the management console with the supervisor account or an Active Directory account rights to change
System Settings; that is, an account in the Console Administration role.
2. Navigate to Preferences | System settings and click Active Directory in the left-hand navigation panel.
3. Under AD Configuration, click Advanced Settings... to specify which sites, domains, domain controllers, or global
catalogs you want the management console to access.
Note: If the Advanced Settings button is not enabled, you must first configure the console for Active
Directory. (See Active Directory Configuration in the MCU Admin Guide.)
4. On the Advanced Active Directory Settings page, select one of the tabs: Sites, Domains, Domain Controllers, or
5. Click Add to specify which site, domain, domain controller, or global catalog you want the console to contact
for all Active Directory related tasks.
6. Enter the site name, domain name, or address, depending on which tab you selected, and click Test connection
to Active Directory.
Note: You must test the connection before you can save!
7. On the Log in to Active Directory page, enter credentials to log into Active Directory and click OK.
8. At the Advanced Active Directory Settings page, click OK to return to System Settings