Yet another time to first byte question

[headfonia]

I've tried optimizing various aspects of the site, with the exception of some stuff like image compression (I can't compress too much), and third party javascript (like the Disqus commenting system), everything is pretty much maxed according to my knowledge:
- caching objects, page, browser, database with APC cache
- setting up expiry headers for cache
- CDN implementation
- minifying CSS and JS

The site is a wordpress site on a dedicated server with a rackspace CDN and the W3TC plug-in. The only thing that I can't seem to shorten is the Time to first byte, as well as well as start to render time. One thing I just noticed is that static image files don't seem to be cached and I wonder why, since I've set them to be cached.

Here is a screenshot of one of the tests:
http://www.webpagetest.org/result/110912...1/details/

And here is one of the last tests I did:
http://www.webpagetest.org/result/110912_5E_1JSZX/



Any help is appreciated. Thanks.