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I tried to log the slow MySQL queries (by enabling the property in the mysql config file), I waited about one day, and I opened the log file...
The file is about 800KB and contains mostly (about 98% of the file) this query :

# Time: 060302 16:35:41
# User@Host: lelombrik[lelombrik] @ localhost []
# Query_time: 21  Lock_time: 0  Rows_sent: 10  Rows_examined: 1763
SELECT u.id AS userid, t.id, t.subject, t.last_poster, t.last_post_id, t.last_post 
FROM punbb_topics AS t 
INNER JOIN punbb_forums AS f ON f.id = t.forum_id 
LEFT JOIN punbb_forum_perms AS fp ON (fp.forum_id = f.id AND fp.group_id=3) 
INNER JOIN punbb_users AS u ON u.username LIKE t.last_poster 
WHERE (fp.read_forum IS NULL OR fp.read_forum=1) 
ORDER BY t.last_post_id DESC 
LIMIT 0, 10;

I didn't look at the PunBB code but I think this should be the index page of the board ?
I don't know why this query is so slow, maybe because of the join with a string... ?

Edit : I tried to rename the extern.php also, but it didn't change anything !

Is there a possibility of fixing this ? By limiting the number of requests or something

Sometimes with 150 users connected, the load is low but it is sometimes very high with 100 users connected (~70 - ~80 - ~120)..