Optimize Query with JOINS and Subqueries - mysql

Question

I want to speed up one of my slower queries.
The problem is that I can't access the outer colum value within a subquery.
What I have:
SELECT r.id AS room_id, r.room_name, coalesce(d.score,0) AS total_messages, d.latest
FROM cf_rooms_time_frames tf
INNER JOIN cf_rooms r on r.id = tf.room_id
INNER JOIN(
SELECT cf.room_id, count(*) as score, max(cf.id) as latest
FROM cf_rooms_messages cf
WHERE EXISTS(
SELECT NULL FROM cf_rooms_time_frames tf
WHERE tf.start <= cf.id AND ( tf.end IS NULL OR tf.end >= cf.id )
AND tf.room_id = cf.room_id AND tf.uid = 8
)
GROUP BY cf.room_id
ORDER BY latest
DESC ) d on d.room_id = r.id
WHERE tf.uid = 8
ORDER BY coalesce(latest, score) DESC LIMIT 0, 20
What I want:
SELECT r.id AS room_id, r.room_name, coalesce(d.score,0) AS total_messages, d.latest
FROM cf_rooms_time_frames tf
INNER JOIN cf_rooms r on r.id = tf.room_id
INNER JOIN(
SELECT cf.room_id, count(*) as score, max(cf.id) as latest
FROM cf_rooms_messages cf
/* line added here */
WHERE cf.room_id = tf.room_id
/* */
AND EXISTS(
SELECT NULL FROM cf_rooms_time_frames tf
WHERE tf.start <= cf.id AND ( tf.end IS NULL OR tf.end >= cf.id )
AND tf.room_id = cf.room_id AND tf.uid = 8
)
GROUP BY cf.room_id
ORDER BY latest
DESC ) d on d.room_id = r.id
WHERE tf.uid = 8
ORDER BY coalesce(latest, score) DESC LIMIT 0, 20
I think the markup explains what the query does.
It searches for "chatrooms" for a given user and orders them by the last message, gets the number of total message which ids are in a given range ( timeframes ), and the last message id.
I don't know why, but the first query returns all rows within the chatmessage table ( cf ) if I can trust EXPLAIN. It delivers the correct results but is kind of slow on a huge table.
I tested the second one with a "hardcoded" room_id and this one was very fast and doesn't "touched" the whole table.