Large user list causing slow performance

Do you have any special roles set up? The user selection form is only supposed to grab contributors and above by default (e.g. "who" => "author")

To clarify, are the subscribers showing up in your users list, or just the contributors, authors, and editors? How many of the latter do you have?

With that many editors, authors and contributors, I think you’re going to have performance issues regardless. Could it be that a small number of users only ever need to be subscribed to posts? If that was the case, you could limit the query to just specific user IDs.

Sure, you can use a filter like this:

add_filter( 'ef_users_select_form_get_users_args', 'efx_users_select_form_get_users_args' );
function efx_users_select_form_get_users_args( $args ) {

	$args['include'] = array(
			1, // User id == 1
		);
	return $args;
}

You’ll want to modify the ‘include’ argument to contain the user IDs for the users you’d like included. Whew, that’s a mouthful 🙂