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.

Optimal indexes for this query:

ALTER TABLE `content_feeds` ADD INDEX `content_feeds_idx_content_type_id` (`content_type`,`id`);
ALTER TABLE `follows` ADD INDEX `follows_idx_follower_id_content_id` (`follower_id`,`content_feed_id`);
ALTER TABLE `posts` ADD INDEX `posts_idx_id` (`id`);

The optimized query:

SELECT
        * 
    FROM
        posts AS p 
    WHERE
        EXISTS (
            SELECT
                1 
            FROM
                content_feeds AS cf 
            WHERE
                cf.id = p.content_feed_id 
                AND cf.content_type = 'youtube' 
                AND EXISTS (
                    SELECT
                        1 
                    FROM
                        follows 
                    WHERE
                        follows.follower_id = 1 
                        AND follows.content_feed_id = cf.id
                )
            ) 
        ORDER BY
            p.id DESC LIMIT 15