Per CloudWatch stats, it varies for machine and region. We have a 6 machines array in three regions receiving round robin traffic from ELB's. It is network bound. However, the CPU graphs say otherwise. Here's the output of three machines in three regions: http://i.imgur.com/nadGmDH.png while their siblings follow the same trend. The working set resides in RAM. The disk and network follow the same path. These are instance store machines sharing as little as possible. Failing over a multi-AZ RDS for maintenance reasons did not change the graphs.