System is very slow with large number of members
To fix this please open phpMyAdmin and go to the newsletter database table.
We need to add some "Index" items to a few database entries in order to speed things up.
To do this, please click on the "member" table, and then click on "Structure" at the top, and then hover over the "More" button next to "email" and chose the "Add Index" option.
Please repeat this process for all the following tables and keys:
member > email
member > unsubscribe_send_id
campaign_member > member_id
member_group > member_id
newsletter_member > member_id
newsletter_member > send_id
newsletter_member > open_time
newsletter_member > bounce_time
newsletter_member > sent_time
member_field_value > member_id