The optimization process and recommendations:

1. Avoid Correlated Subqueries In Select Clause (modified query below): The aggregation function located in a subquery inside the SELECT clause, is executed once for every matched row. Extracting this subquery to a temporary table will improve performance significantly.
2. 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.

Optimal indexes for this query:

ALTER TABLE `es_temp1` ADD INDEX `es_temp1_idx_user_id` (`USER_ID`);
ALTER TABLE `es_temp2` ADD INDEX `es_temp2_idx_user_id` (`USER_ID`);
ALTER TABLE `es_temp3` ADD INDEX `es_temp3_idx_user_id` (`USER_ID`);
ALTER TABLE `es_temp4` ADD INDEX `es_temp4_idx_user_id` (`USER_ID`);

The optimized query:

SELECT
        u.ID,
        es_temp1.interesting_posts,
        es_temp2.boring_posts,
        es_temp3.interesting_comments,
        es_temp4.boring_comments 
    FROM
        USERS u 
    LEFT JOIN
        es_temp1 
            ON es_temp1.USER_ID = u.ID 
    LEFT JOIN
        es_temp2 
            ON es_temp2.USER_ID = u.ID 
    LEFT JOIN
        es_temp3 
            ON es_temp3.USER_ID = u.ID 
    LEFT JOIN
        es_temp4 
            ON es_temp4.USER_ID = u.ID