We recommend that you use iManager for user management in existing environments, and MMC (Microsoft Management Console) for other operations. Follow the guidelines given below to improve domain performance:
When a user is a member of several groups, the login time can increase. Avoid adding users to too many groups.
Having large number of groups in a domain might result in an increase in resource utilization, as a result of calculation of the effective group membership. Avoid adding too many groups.
Avoid adding users directly to Universal groups. Instead, place users in global groups and add global groups in universal groups. Adding users directly to universal groups can delay user login.
For more information, see Securing Active Directory Administrative Groups and Accounts.