The optimization process and recommendations:

1. Avoid Selecting Unnecessary Columns (query line: 28): 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: 12): 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.

Optimal indexes for this query:

ALTER TABLE `TableB` ADD INDEX `tableb_idx_workid_dateb` (`WorkId`,`DateB`);
ALTER TABLE `TableB` ADD INDEX `tableb_idx_flag` (`Flag`);

The optimized query:

WITH c AS (SELECT
        a.WorkId,
        a.DateA,
        b.User AS User1 
    FROM
        TableA a 
    INNER JOIN
        TableB b 
            ON a.WorkId = b.WorkId 
            AND a.DateA = b.DateB),
    c1 AS (SELECT
        c.*,
        b.DateB,
        b.User AS User2,
        ROW_NUMBER() OVER (PARTITION 
    BY
        b.WorkId,
        c.DateA 
    ORDER BY
        b.DateB) AS rn 
    FROM
        c 
    LEFT OUTER JOIN
        TableB b 
            ON c.WorkId = b.WorkId 
            AND b.Flag = 'N' 
            AND b.DateB > c.DateA) SELECT
            * 
    FROM
        c1 
    WHERE
        c1.rn = 1