Map all the select queires to slave

Hi,

*I have a heavy traffic sites approximately 50k users per day and it is a Indian News portal.
*The site is running on Druapl 7.22 and Mysql 5.5
* The site has a Load Balancer ,2 Application servers with 100GB of Hard Disk and 20GB of RAM each and 2 DB Servers(Master/Slave).
*The front end caching mechanism is varnish with 10GB of Storage Size and CDN.

The issue is even though we have these much things, the slow queries affects the sites performance and every day the sites goes down. So I am planning to map all select queries to Slave or any one suggest to get rid of this issue.

Sorry About My English...