how to improve webpage speed in drupal

I have a big content we开发者_如何学JAVAbsite which is hosted on rackspace in USA. The site downloads time is close to 12 seconds despite using CDN module, image jss etc. as per Pagespeed, website gets close to 85/100 and on Yslow it gets B grade. How can I bring website speed to less than 3 seconds. I have already installed APC

Memcached and Varnish will drastically improve your page load speeds. This presentation from 2bits introduced me to the world of server optimization. Also check out the lullabot series on improving performance.