The optimization process and recommendations:

1. Avoid Subqueries (query line: 8): We advise against using subqueries as they are not optimized well by the optimizer. Therefore, it's recommended to join a newly created temporary table that holds the data, which also includes the relevant search index.
2. Avoid Subqueries (query line: 29): We advise against using subqueries as they are not optimized well by the optimizer. Therefore, it's recommended to join a newly created temporary table that holds the data, which also includes the relevant search index.
3. Avoid Subqueries (query line: 54): We advise against using subqueries as they are not optimized well by the optimizer. Therefore, it's recommended to join a newly created temporary table that holds the data, which also includes the relevant search index.
4. 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 `subset_all` ADD INDEX `subset_all_idx_ratingdatetime` (`RatingDateTime`);

The optimized query:

SELECT
        ((SELECT
            sum(LM) 
        FROM
            (SELECT
                sum(value) LM 
            FROM
                (SELECT
                    subset_all.segmentid,
                    subset_all.rating,
                    subset_all.RatingDateTime,
                    subset_all.value,
                    row_number() OVER (PARTITION 
                BY
                    segmentid 
                ORDER BY
                    subset_all.RatingDateTime DESC) AS rn 
                FROM
                    subset_all 
                WHERE
                    subset_all.RatingDateTime < date('2014-10-20')) 
            WHERE
                rating >= 7 
                AND rn = 1 
            UNION
            ALL SELECT
                sum(value) LM 
            FROM
                (SELECT
                    subset_all.segmentid,
                    subset_all.rating,
                    subset_all.RatingDateTime,
                    subset_all.value,
                    row_number() OVER (PARTITION 
                BY
                    segmentid 
                ORDER BY
                    subset_all.RatingDateTime DESC) AS rn 
                FROM
                    subset_all 
                WHERE
                    subset_all.RatingDateTime < date('2014-10-20')) 
            WHERE
                rating >= 8 
                AND rn = 1
            )
    ) + (
        SELECT
            sum(LM) 
        FROM
            (SELECT
                sum(value) LM 
            FROM
                (SELECT
                    subset_all.segmentid,
                    subset_all.RatingDateTime,
                    subset_all.value,
                    row_number() OVER (PARTITION 
                BY
                    segmentid 
                ORDER BY
                    subset_all.RatingDateTime DESC) AS rn 
                FROM
                    subset_all 
                WHERE
                    subset_all.RatingDateTime < date('2014-10-20')) 
            WHERE
                rn = 1
            )))