By Owner impact:
Selecting ‘By Owner’ auditing for many mailboxes can produce a very large number of events. This adversely affects Change Auditor auditing and in severe cases the performance of the Exchange Server itself. In extreme cases, Outlook® connections may be slowed or dropped. Select owner auditing for at most only a small number of critical mailboxes.
‘By Owner’ events are disabled by default. Therefore, it is necessary to enable these events from the Audit Events page on the Administration Tasks tab before Change Auditor will process these them.
Auditing normal mailboxes where access permission is granted to many delegates (more than 10), can produce large numbers of non-owner events. This will adversely affect Change Auditor auditing and in severe cases, the performance of the Exchange Server itself. If these mailboxes need to be audited, add them to the Shared Mailbox list (User Defined tab) to reduce unwanted non-owner events and to improve performance. See Managing Shared Mailboxes for more information.