Currently there is a configuration for event import which has the following online help description:
"Note: This setting is provided for policy engine diagnostic purposes only. We strongly recommend that you do NOT change this setting.
This setting specifies whether the policy engine connects to an LDAP directory to retrieve e-mail address information and distribution list members. If it set to False, policy engines cannot retrieve details such as address aliases from the LDAP directory. Also, they cannot expand distribution lists and so cannot apply policy for individual list members. "
However, the lookup is effectively looking up 'everything' only to bring back a little required data and can be 'expensive' on the GC (Global Catalog) when there are millions of events per day.
This enhancement request is to have the LDAP Lookup as a lightweight query that should help achieve a minimal Global Catalog load.