why is searching in an ACF post object field so slow?

If you have several hundred or thousand posts that must be searched through, changing the ACF post object to only search in titles is a huge performance enhancement.

In functions.php:

function add_search_only_titles_filter($args, $field, $post_id) {
    add_filter('posts_search', '__search_by_title_only', 500, 2);
    return $args;
}

function __search_by_title_only( $search, &$wp_query ) {
    global $wpdb;

    if ( empty( $search ) )
        return $search; // skip processing - no search term in query

    $q = $wp_query->query_vars;    
    $n = ! empty( $q['exact'] ) ? '' : '%';

    $search =
    $searchand = '';

    foreach ( (array) $q['search_terms'] as $term ) {
        $term = esc_sql( like_escape( $term ) );
        $search .= "{$searchand}($wpdb->posts.post_title LIKE '{$n}{$term}{$n}')";
        $searchand = ' AND ';
    }

    if ( ! empty( $search ) ) {
        $search = " AND ({$search}) ";
        if ( ! is_user_logged_in() )
            $search .= " AND ($wpdb->posts.post_password = '') ";
    }

    return $search;
}

add_filter('acf/fields/post_object/query', 'add_search_only_titles_filter', 10, 3);