Since the upgrade from WPMU 2.9.2 to WP 3.0.1, we've had problems with slowness. There was a big improvement after I ran a repair and optimize on all the MySQL tables, but we still notice that certain blogs run more slowly than others.
I noticed that for the blogs running more slowly, the wp_x_options table is especially large in size - e.g. several hundred KB in size rather than < 100 KB in size. And the large entries in the table are generally named "_transient_feed_[some long hex]".
It's hard for me to tell what these are for - I think they're for caching something, but the blogs with these do not have RSS feeds in their widgets or anything like that. Google searches about these are mostly turning up "oh noes I am haxx0red" posts, but I don't think that's the issue here.
If I delete these caches and run an optimize on the options table, the blog will temporarily speed up again. So clearly they're evil! No, I suppose not. But can anyone explain to me what the transient_feed entries are for, and whether they might be causing this issue, and if so, how I can tame them? Thanks.