I'm not sure to fully get your point. The "performer name" is the user who actually did the work because it was assigned to it. And sometimes it is "System" for example when an SLA action is triggered.
In attached screenshot you can verify once Case has been approved at any stage there will be some rows added in Audit log where performed by will be User name who has approved this case.
If you see in screen shot there are 4 rows added in this case when case has been approved. Here i want to display user name only for "Case has been approved at Pre-Evidence" and in remaining rows performed by should display "System".