如果要测试自定义SQL以查看其对加载时间的影响，可以尝试以下查询交换：

/**
 * Restrict the potential slow query in the meta_form() to the current post ID.
 *
 * @see http://wordpress.stackexchange.com/a/187712/26350
 */

add_action( \'add_meta_boxes_post\', function( $post )
{
    add_filter( \'query\', function( $sql ) use ( $post )
    {
        global $wpdb;
        $find = "SELECT meta_key
                 FROM $wpdb->postmeta
                 GROUP BY meta_key 
                 HAVING meta_key NOT LIKE \'\\\\\\_%\'
                 ORDER BY meta_key 
                 LIMIT 30";
        if(    preg_replace( \'/\\s+/\', \' \', $sql ) === preg_replace( \'/\\s+/\', \' \', $find )
            && $post instanceof WP_Post  
        ) {
            $post_id = (int) $post->ID;
            $sql  = "SELECT meta_key
                     FROM $wpdb->postmeta
                     WHERE post_id = {$post_id}
                     GROUP BY meta_key
                     HAVING meta_key NOT LIKE \'\\\\\\_%\'
                     ORDER BY meta_key
                     LIMIT 30";
        }
        return $sql;
    } );                                                            
} );

这里我们交换了整个查询，但我们也可以调整它以支持动态限制。

希望您可以根据自己的需要进行调整。

更新：

SELECT meta_key 
FROM wp_postmeta 
GROUP BY meta_key 
HAVING meta_key NOT LIKE \'\\\\_%\' 
ORDER BY meta_key 
LIMIT 30

SELECT DISTINCT meta_key
FROM wp_postmeta
WHERE meta_key NOT BETWEEN \'_\' AND \'_z\'
HAVING meta_key NOT LIKE \'\\_%\'
ORDER BY meta_key
LIMIT 30;

如果浏览函数的源代码，您会发现：

$keys = apply_filters( \'postmeta_form_keys\', null, $post );
if ( null === $keys ) {
    ...      
}

add_filter(\'postmeta_form_keys\', function(){
    return [\'your_meta_key\'];
});

你能试试这个吗。这不是一个解决方案，而是一个临时解决方案。

// disable big slowdown http://wordpress.stackexchange.com/questions/187612/admin-very-slow-edit-page-caused-by-core-meta-query
function dj_limit_postmeta( $string, $post ) {
    return array(null);
}
add_filter( \'postmeta_form_keys\', \'dj_limit_postmeta\', 10, 3 );