@mpaturi
I like web services because they run completely independent of the web app (asp in this case).
If you are running a .NET web service, it will be compiled and run separately from the web app. The communication causes a brief lag, but if the service takes a while to run it will run faster like this.
If you use a dll ASP will run it as a server object. This will make the server run slower in the long run, so I wouldn't use that if the calculations it performs are especially long or if it takes a lot of memory. but if it is a fairly simple calculation, that would be faster because you don't have to wait to communicate with a web service.
Jared