[]Using where; Using filesort [/]
Your query doesn't use an index so it has to do a additional full table scan to order the results. But the query is executed quite fast at the moment.
The results will only be cached if the posts table hasn't changed in the mean time. But on an active boards, it changes nearly every second.
It would be a good idea to log all slow queries (option in your my.cnf), it usually helps to determine the bottleneck.
