Optimizing Nested Query in SQL Server

My friend has been asked to optimize the SQL query for a forum application. Basically the Query is used to list the thread as well as the last person who posted on the forum. He and me tried to optimize that and we can fasten up the query up to only 13 secs which is not really well 🙁 . then the sql guru is coming to help him and show him the better way in writing the query and it performs only 1 sec. I thought it’s gonna be a good lesson to be shared it on my blog. There you go:

--Non Optimized Version
SELECT
	ForumTopic.ForumTopicID, ForumTopic.Title, Members.Username AS MemberName,
	ForumTopic.Views, ForumTopic.Replies,
	(
		SELECT TOP 1
			ForumPost.EnteredDate
		FROM
			ForumPost
		WHERE
			ForumPost.ForumTopicID = ForumTopic.ForumTopicID
		ORDER BY
			ForumPost.ForumPostID DESC
	) AS LastPostEnteredDate,
	(
		SELECT TOP 1
			Members.Username
		FROM
			ForumPost
		INNER JOIN
			Members
		ON
			ForumPost.EnteredBy = Members.MemberID
		WHERE
			ForumPost.ForumTopicID = ForumTopic.ForumTopicID
		ORDER BY
			ForumPost.ForumPostID DESC
	) AS LastPostMemberName
FROM
	ForumTopic
INNER JOIN
	Members
ON
	ForumTopic.EnteredBy = Members.MemberID
WHERE
	ForumID = @ForumID
AND
	ForumTopic.Valid = 1
ORDER BY
	LastPostEnteredDate DESC

--optimized version
SELECT
	ForumTopic.ForumTopicID,
	ForumTopic.Title,
	Members.UserName as MemberName,
	ForumTopic.Views,
	ForumTopic.Replies,
	ForumPost.EnteredDate as LastPostEnteredDate,
	Members1.Username as LastPostMemberName
FROM
	(
		SELECT
			ForumTopicID, MAX(ForumPostID) AS ForumPostID
		FROM
			ForumPost
		WHERE
			ForumTopicID IN (
				SELECT
					ForumTopicID
				FROM
					ForumTopic
				WHERE
					ForumTopic.ForumID = @ForumID
				AND
					ForumTopic.Valid = 1)
		GROUP BY
			ForumTopicID
	) RecentPost
INNER JOIN
	ForumPost
ON
	ForumPost.ForumPostID = RecentPost.ForumPostID
INNER JOIN
	ForumTopic
ON
	ForumTopic.ForumTopicID = RecentPost.ForumTopicID
INNER JOIN
	Members
ON
	ForumTopic.EnteredBy = Members.MemberID
INNER JOIN
	Members Members1
ON
	ForumPost.EnteredBy = Members1.MemberID
ORDER BY
	ForumPost.EnteredDate DESC