Well, @alec - your suggestion works. I was hoping to still allow sorting during the user's session and return the value to default at logout, but it's not a show-stopper. With the following snippet in my config, users can't sort on any columns, but the login process is **WAY** faster than before, which is a much bigger deal than sorting!
Thanks for the suggestion!
//////////////////////////////////////////////
// ----------------------------------
// USER INTERFACE
// ----------------------------------
// default messages sort column. Use empty value for default server's sorting,
// or 'arrival', 'date', 'subject', 'from', 'to', 'fromto', 'size', 'cc'
$config['message_sort_col'] = '';
// default messages sort order
$config['message_sort_order'] = '';
$config['dont_override'] = array('message_sort_order','message_sort_col');