The back end of the Wordpress installation over at Rational Review News Digest started running really slow awhile back. The slowdown was, for the most part, noticeable when I'd hit "publish" or "schedule" on a post.
Silly me, I didn't think to check the plugins, because I hadn't installed any new ones recently. I messed around with various cache settings and database optimizations and so forth, to no effect.
A few days ago, I decided that maybe the site was just getting too damn big at ~30,000 posts, and started deleting old material (it's a news and commentary roundup/excerpt/link site -- the old stuff may be mildly useful for researchers trying to find stuff, but it's not like there's a bunch of original material there). But that went VERY slowly, too, even using the "mass delete" plugin (it kept timing out).
I finally got tired enough of it yesterday to start selectively shutting down the plugins I could afford to do without (some of them are just absolutely required to get things the way they need to be), and discovered that when I turned off the Google sitemap plugin, bam -- things started running at normal speed again.
My guess -- and it's just a guess -- is that the plugin was re-indexing the whole damn site every time I added, deleted, etc. a post, and that at a certain point there was just too much material for it to handle in a timely manner.
In any case, if you have a Wordpress site with a lot of posts, and if it suddenly starts running like a snail with a stomach ache, and if you have that particular plugin running, try turning it off and see what happens.