$args = array(
                'post_type'      => $post_types,
                'posts_per_page' => 200, // 200 sounds like a safe place for performance?
                'fields'         => 'ids',
                'meta_query'     => array(
                    array(
                        'key'     => 'wpf-settings',
                        'compare' => 'EXISTS',
                    ),
                ),
            );