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. 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.
3. Prefer Inner Join Over Left Join (modified query below): We identified that one or more left joined entities (e.g. `forumTopic`) 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.
4. Prefer Inner Join Over Left Join (modified query below): We identified that one or more left joined entities (e.g. `forumPost`) 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.

Optimal indexes for this query:

ALTER TABLE `forumCategory` ADD INDEX `forumcategory_idx_rewrittenname_active` (`rewrittenName`,`active`);
ALTER TABLE `forumPost` ADD INDEX `forumpost_idx_active_forumtopicid` (`active`,`forumTopicId`);
ALTER TABLE `forumTopic` ADD INDEX `forumtopic_idx_active_forumcateg_id` (`active`,`forumCategoryId`,`id`);
ALTER TABLE `user` ADD INDEX `user_idx_id` (`id`);

The optimized query:

SELECT
        forumTopic.*,
        COUNT(forumPost.id) AS postCount,
        MAX(forumPost.date) AS lastPostDate,
        (SELECT
            u1.name 
        FROM
            user AS u1 
        WHERE
            u1.id = forumPost.posterUserId 
            AND forumPost.date = MAX(forumPost.date)) AS lastPostUserName,
        (SELECT
            u2.name 
        FROM
            user AS u2 
        WHERE
            u2.id = forumPost.posterUserId 
            AND forumPost.date = MIN(forumPost.date)) AS starterUserName 
    FROM
        forumCategory 
    INNER JOIN
        forumTopic 
            ON forumCategory.id = forumTopic.forumCategoryId 
    INNER JOIN
        forumPost 
            ON forumPost.forumTopicId = forumTopic.id 
    WHERE
        forumCategory.rewrittenName = 'someforumcategory' 
        AND forumCategory.active = 'Y' 
        AND forumTopic.active = 'Y' 
        AND forumPost.active = 'Y' 
    GROUP BY
        forumTopic.id 
    ORDER BY
        forumPost.date ASC