Deleting a user on a medium size forum with zero messages takes multiples of seconds. Backend is mysql 5.7.29 / MyISAM. Looking into slow queries log, the following queries take seconds:

UPDATE bbs_posts
SET post_edit_user = 1
WHERE post_edit_user = 103246;

UPDATE bbs_posts
SET post_delete_user = 1
WHERE post_delete_user = 103246;

Adding two indexes makes user delete operation sub-second as expected. Suggesting adding the following indexes to the posts table:

create index post_edit_user on ...posts (post_edit_user);

create index post_delete_user on ...posts (post_delete_user);

From trade-off perspective, since we optimize "for read" small additional tax on insert should be justifiable.