>I am using HTTP load testing tool to stress test my asp.net 2.0 application. I faced problem that, when I have 1 user access to the system (currently only test on login.aspx), it is very fast ~ 1-2 secs. However, when I run with 10 virtual users at the same time, it takes 10-12 secs for each.

At first, one to 2 seconds to access a login is not fast. For the Web, especially if you are targeting at heavy traffic, which seems the case here as you are stress testing it, you should target at 0.1 second. Now, even if you would bring it down to 0.5 second, this would mean, in average, you should be able to process four times more hit on your server on a daily basis without having to invest on the infrastructure.

I wouldn't know what is causing the latency on multiple users. This is something specific to your design.