When Running Query Slows down Entire Board

[atrljoe]

Hey everyone, when ever I run this query

                SELECT threadid, title, lastpost, postusername, lastpostid, forumid, replycount, prefixid      
                FROM thread   
                WHERE thread.visible = 1   
                AND thread.forumid = 67  
                ORDER BY thread.lastpost DESC   
                LIMIT 5

My board slows down a lot, server loads get to 60-80, and the slow queries go off the charts. My question is what is causing this, all I am trying to do is get the last 5 threads from that forum.

Thanks