Its well worth reading the article up on IIS.NET about the new architecture of IIS 7. This background will provide you with a far better way to troubleshoot IIS.

In particular having a clear understanding on the request flow plus any thread pools etc will really help to understand where bottlenecks could be occurring. A good starting point is here:

http://www.iis.net/articles/view.aspx/Extending-IIS7/Getting-Started/Introduction-to-IIS-7-Architecture?Page=1
http://www.iis.net/articles/view.aspx/Extending-IIS7/Getting-Started/Introduction-to-IIS-7-Architecture?Page=2
http://www.iis.net/articles/view.aspx/Extending-IIS7/Getting-Started/Introduction-to-IIS-7-Architecture?Page=3
http://www.iis.net/articles/view.aspx/Extending-IIS7/Getting-Started/Introduction-to-IIS-7-Architecture?Page=4

Page 4 has some flow diagrams which make things a little clearer.