[Mailman-Developers] Answering my own question

Adding a "save_list=0" (or just a fourth argument '0') to the two calls to SetUserOption in admin.py speeds up that "set one option" flag a *whole* lot. It might be that similar "optimizations" to the other forms processing (DeleteMember, primarily) are a bit unsafe, but it seems like a huge win for the SetUserOption calls, and not very unsafe, since the list will be saved after processing all users anyway.