Speed up my Query please

[Swedie]

I have a quite large query to perform towards the vB database post table.

I want to pull out the last 10 posts, but I do not want to include certain threadid's and forumid's.

Right now the query is like this:
mysql_query("SELECT * FROM post WHERE threadid NOT IN($excludethreadid) AND forumid NOT IN($excludeforumid) LIMIT 10");
$excludethreadid is set with a bunch of threadids (into the hundreds, maybe toward a thousand).
$excludeforumid has a bunch of forumids not to be included in the query result.

Thing is that this query, when I have around 600+ users, is taking an extremely large amount of the CPU power and queing up requests and bogging down the whole system.

Is there a better way than my version above to query the post table and exclude certain preset id's in the two fields threadid and forumid?

Would appreciate help in this matter.