I switched to a new webhost that uses an opcode cache and that has made a huge difference.

I don’t know what version of PHP Speed WP, but the version I’m running shaved 17% off load times, compresses the CSS and JS, and automatically moves the CSS to load first, and JS last. It has reduced the db calls. I blogged about it here – http://itjuggler.ollis.id.au/2009/02/16/wordpress-on-speedy-wp/

I think this does something similar by creating a file based backend for the built in wordpress caching framework. It sounds like this file-based mechanism was built-in to wordpress 2.1 and removed in 2.5 because a lot of people had complaints from their shared hosting providers that it was causing excessive IO.

Assuming they are similar, it definitely makes a difference on my web host, but the query time was only 10% of so of total request processing time before, so eliminating it doesn’t have a big impact on overall responsiveness. What’s really needed is something that can accelerate the startup time, particularly the loading of plugins.