Help Configuring Nginx's Caching For High Traffic Yii App

I am looking for examples or advice on:

  • how to configure Nginx FastCGI caching for a Yii application that is running fine on Nginx with full page filecaching and xcache enabled, but still feels slow.

  • understanding how Yii-level caching and Nginx (server-level) caching should interact for optimal performance.

Any help would be appreciated.