Adriana,
I do not know if the information requested, but we have historically had similar problems with different settings.
If you are using EEM as a user repository, how many users you have registered?
If you are using EEM with AD you can tell me:
1. what is the connection settings? , How does the customer domains?
2. You have configured the connection for LDAP (port tcp 389) or Global Catalog (port tcp 3268)?
Our experience, generally, the problems are of slowness between AD and EEM
Juan.