Slow AD discovery performance collecting group membership after upgrading to version 3.5 or greater. Discovery may take days to complete when it was taking only hours to complete before the upgrade.
Starting with version 3.5, Enterprise Reporter retrieves group membership information from foreign domains by querying each domain individually for every user to determine their respective group membership.
As a workaround, under discovery scope you may uncheck the option Include members from foreign domains to skip this during discovery. If you need group membership information from foreign domains, then you may create an AD discovery for each domain to collect that information separately.