> If you’re spending more time during each request doing database work rather than PHP work, then you’ve got a problem that needs to be fixed.

For most webapps, I/O typically should be the bottleneck. This holds especially true for wordpress, which has nothing computationally heavy to do, and is...rather liberal with the way it queries the database. Certainly the large installations I've managed spent most time doing I/O (either in the database or memcached.)