The asset pipeline which was introduced in Rails 3.1, has been extracted to a separate gem called sprockets-rails. Performance for assets compiling has been improved. There is a open pull request to merge turbo-sprockets-rails3 to sprockets-rails, which would improve its performance further more.
With the commit c82cf81f00f Action and Page caching has been extracted from Action Dispatch as `actionpack-action_caching` and `actionpack-page_caching` gems
Speeds up your Rails 3 assets:precompile by only recompiling changed files http://t.co/IU8fonq3 #rails3 — Akhil Bansal (@bansalakhil) October 3, 2012