Again. I would recommend to enable 'automatic user creation' for the facilities you want to use. Users created this way have all email addresses imported. If you feel that the ASG has a very high load when a lot of users are created at the same time* that is the point where you want to try user prefetch. (Of course, if you want to use user prefetch for any other reason, feel free )

* a lot of users means >100 new users log in at roughly the same time, and have their user objects created