How to configure IIS for intranet only

We’re running IIS 4.0 on a NT 4 server.
We have a browser based application (EATEC) and the application runs faster when we connect the server to the internet.

Without it, it takes over a minute to login using the intranet (wired ethernet connection). With the internet connection enabled on the server, login takes 2-3 seconds.

Obviously, IIS is looking for something on the internet but what? Or is it the application?

All services (ftp,smtp) except the web service are stopped.

Any thoughts?