Speed issue happen when sending message emails to users email address, not when sending message. Most of the host does not allow bulk emails, most of the time they have limit per hour.

It is better if you send announcement as that will queue email sending and send in interval. Department can go to Front-End page (where [front-end-pm] shortcode is) and click “New Announcement”.

Let me know if it works for you.

Announcement can be send to role. If those 800 user have separate role then this can easily be targeted. Or you can create a temporary role, assign this role to those 800 users then send announcement to that role.

Currently you can use fep_action_announcement_after_added hook and remove those users using FEP_Participants::init()->delete( $announcement_id, $participant_id );

To ease this steps i have added a new filter here. Next version you will get this filter. Using this filter you will be able to remove those users before adding to database which will significantly improve performance.