The feedback Module calls in show_entries.php lines129/131 '$students = feedback_get_complete_users($cm->id);' for users which are allowed to complete the feedback. But this call returns a array of all users of Moodle either the feedback is on the frontpage or in a normal course. For our platform with 17000 users the memory consumption is about 250MB.
Replacing this call with a call to: '$students = get_records_sql("select user.* from mdl_user user, mdl_feedback_completed WHERE user.id=userid AND feedback=$feedback->id order by lastname");' which returns all users which filled in a feedback returns only the necessary users (and perhaps some old users not more in the course).
I think this is a bug either in the core or the feedback module. With this change the script consumes only 17MB of memory.
If you select only a group everything is okay with the original code.