I'm heavily depending on ASP.net performance counters to keep track of web application activity. After years of using IIS URL Rewrite to rewrite ASP.net 2.0 applications with no issue, we upgraded to ASP.net 4.0 and suddenly noticed a 100% jump in number of requests processed per second. I did a lot of search to understand why and could not find anything. I was almost convinced that this jump is due to performance improvements caused by .net framework upgrade, until I tested the app without URL Rewrite and noticed the difference.
I used ASP.net routing to replace URL Rewrite and counters are back to normal again. To learn more about the differences between the URL Rewrite and ASP.net Routing, read this.