It's a complete myth that closing long threads helps the forums run smoothly. The forum software doesn't store a thread as a whole unit; each post has a unique key and exists independently of a thread, meaning you can make a thread as arbitrarily short or long as you want and it won't speed up or slow down the forum.
The only thing that significantly slows down a forum is search, which is why you disable it for guests, impose a time-limit between searches, and don't allow searches for less than 5 characters. However, search efficiency still remains independent of thread lengths.
Funny, I was thinking we needed less of you sheep rooters.