Zkuste přesunout IN do klauzule FROM jako inline odvozenou tabulku
SELECT
id_topic, id_member_comment, pd.username, dt_post
FROM
(
SELECT MAX(last_id_comment) AS id_comment
FROM forum_topic
GROUP BY cat_id
) AS foo
JOIN
forum_comment c ON foo.id_comment = c.id_comment --AND a cat_id join too?
LEFT JOIN
persondata pd ON c.id_member_comment = pd.id_member;