MySql Query very slow

You must use a temporary table and an alias:

SELECT SQL_CALC_FOUND_ROWS * 
FROM meruca_posts 
WHERE ID NOT IN (SELECT ID 
            FROM (
                    SELECT object_id as ID
                    FROM meruca_term_relationships
                    WHERE term_taxonomy_id IN (14)
                )
            AS temp_object)
AND post_type="post" 
AND post_status IN ('publish','private') 
GROUP BY ID ORDER BY post_date DESC LIMIT 0, 10