The article take a look at how we can use OEM as a jumping off point to drill down and find root causes for performance problems in real-time, specifically looking at monitoring user activity, monitoring instance activity, and monitoring host activity. Each of these together brings a nice and total view of those things that can cause database performance issues.

http://www.dbasupport.com/oracle/ora11g/Looking-at-Real-Time-Database-Performance.shtml