An excellent blog explaining the ExecutionLog2 view within Reporting Services has been written by Robert Bruckner at: http://blogs.msdn.com/robertbruckner/archive/2009/01/05/executionlog2-view.aspx
Quoted from Robert:
Before you can optimize particular reports or your entire system, you need metrics and understand what they tell you. In this posting, I want to focus on how to effectively interpret and utilize the data present in the new ExecutionLog2 view in the Reporting Services 2008 catalog database.
Denny Lee 