Hi @Nitruc
Quite simply, the ‘wp_insert_post_data’ action is run BEFORE ACF save’s its data. This is why you get a ‘page load delay’.
Your best bet is to use the acf/update_value action to update the post title.
You can read about this action on the ACF docs page + there are already a few topics regarding updating a post’s title via an ACF field!
Thanks
E
