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:

CREATE INDEX categories_article_idx_category_id_article_id ON "categories_articles" ("category_id","article_id");
CREATE INDEX comments_idx_is_visible_is_banned_posted_dat ON "comments" ("is_visible","is_banned","posted_date" desc);
CREATE INDEX comments_idx_posted_date ON "comments" ("posted_date" desc);

The optimized query:

SELECT
        * 
    FROM
        comments co 
    JOIN
        categories_articles ca 
            ON ca.article_id = co.article_id 
    WHERE
        ca.category_id = 1 
        AND co.is_visible = TRUE 
        AND co.is_banned = FALSE 
    ORDER BY
        co.posted_date DESC LIMIT 20