A new chapter has just been added to the symfony book : How to speed up a site with the caching system. It explains in detail how to setup HTML cache to transform your fast symfony application into a light speed web-app at no cost. You should definitely have a look at it if you are interested in application performance and optimization.
You need a version 0.5.134 (or more) of the symfony package to be able to use the features described in this chapter, so make sure you upgrade your installation before trying them.
In addition, the symfony website has a new feature: you can now add comments to every chapter in the book, tutorial, or article in the website. Give us your feedback!