Black Hole Effect

Have you ever tried to log on to a terminal server and, after entering your credentials, been forced to stare at a grey screen for a lengthy period of time wondering what the machine might actually be doing? Of course you have, along with a few million other terminal server users. Being a technical guy (you would not be reading this otherwise) you have checked CPU / memory / hard disk utilization and the current session count when users complain that logons are slow. You will probably have noticed that all relevant metrics are in the green and logons are the slower, the more users try to log on to a server concurrently. It turns out that parallel logons are the root cause of the problem. Why?