Often, small changes to your code can have a large effect on the performance of your application. Unfortunately, it's difficult to know exactly where these bottlenecks will occur. One of the best ways of finding out how your application will perform with a large set of concurrent users is to simulate that exact situation. Microsoft provides a free tool called the Microsoft Web Application Stress Tool for generating user load. You can download it from http://webtool.rte.microsoft.com/.