MAX Date Query Takes Too Long To Run

[HairyArse]

Hi guys, I have a query which seems to be slowing down my forum.

The URL is http://www.allaboutthegames.net/forum/index.php and the query that I run to generate the forum index is taking almost a second to run which seems quite slow to me. Is there a more efficient way of querying the same information?

SELECT a.thread_title,a.thread_id, a.thread_stickied, a.thread_locked, MAX(b.reply_date) AS last_post_on,c.category_name,c.category_id,d.user_id AS thread_started_by_id, d.username AS thread_started_by FROM tbl_forum_threads a, tbl_forum_replies b, tbl_forum_categories c, tbl_users d WHERE a.thread_category_id = c.category_id AND a.thread_id = b.thread_id AND a.thread_author = d.user_id GROUP BY a.thread_id ORDER BY a. thread_stickied ASC , last_post_on DESC

[HairyArse]

What also might not be helping is that I've also got a second nested query that runs for every row of the above:

SELECT a.reply_author, b.username FROM tbl_forum_replies a, tbl_users b WHERE a.reply_date = '$last_post_on' and a.thread_id = '$thread_id' AND a.reply_author = b.user_id

Where last_post_on and thread_id are variables set from the above query.

Is there any way of combining these two queries into one?