Currently, to search a single User in LDAP, Crowd concatenates the Connector User Object Filter with the query to search the username:

Connector User Object Filter: (&(objectCategory=Person)(sAMAccountName=*))
User Query: (sAMAccountName=user_name)

Crowd Search: (&(&(objectCategory=Person)(sAMAccountName=*))(sAMAccountName=user_name))

It may be better to stop concatenating the default User Object Filter to the User name filter because of the wildcard (sAMAccountName=*).

We believe that AD is smart enough to maximize the query performance in this case, however, it must be investigated to find alternatives and improve it on our side.