The optimization process and recommendations:

1. Avoid Selecting Unnecessary Columns (query line: 2): Avoid selecting all columns with the '*' wildcard, unless you intend to use them all. Selecting redundant columns may result in unnecessary performance degradation.
2. Avoid Selecting Unnecessary Columns (query line: 6): Avoid selecting all columns with the '*' wildcard, unless you intend to use them all. Selecting redundant columns may result in unnecessary performance degradation.
3. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
4. Prefer Inner Join Over Left Join (modified query below): We identified that one or more left joined entities (e.g. `wp_terms`) are used in the 'where' clause, in a way that allows to replace it with an optimized inner join. Inner joins can be fully optimized by the database, while Left joins apply limitations on the database's optimizer.
5. Sort and Limit Before Joining (modified query below): In cases where the joins aren't filtering any rows, it's possible to sort and limit the amount of rows using a subquery in the FROM clause, before applying the joins to all other tables.

Optimal indexes for this query:

ALTER TABLE `wp_posts` ADD INDEX `wp_posts_idx_post_date` (`post_date`);
ALTER TABLE `wp_term_relationships` ADD INDEX `wp_relationships_idx_term_id` (`term_taxonomy_id`);
ALTER TABLE `wp_term_taxonomy` ADD INDEX `wp_taxonomy_idx_term_id` (`term_id`);
ALTER TABLE `wp_terms` ADD INDEX `wp_terms_idx_term_id` (`term_id`);

The optimized query:

SELECT
        p.*,
        t.term_id 
    FROM
        (SELECT
            p.*,
            p.ID AS p_ID 
        FROM
            wp_posts p 
        ORDER BY
            p.post_date DESC LIMIT 10) p 
    LEFT JOIN
        wp_term_relationships rel 
            ON rel.object_id = p.p_ID 
    LEFT JOIN
        wp_term_taxonomy tax 
            ON tax.term_taxonomy_id = rel.term_taxonomy_id 
    INNER JOIN
        wp_terms t 
            ON t.term_id = tax.term_id 
    WHERE
        t.term_id = 6 LIMIT 10