It's not magic. The more you place in a level, the slower it will run.
There are various tricks that Lee is using to keep the speed up, such as not rendering rooms or object that are outside your view, or reducing the number of updates used by the AI as AI controlled objects are further away from the player - This all helps, but it won't solve the problems caused by dropping 50 monsters into a single room.
*** Coming soon - Network Plug-in - Check my site for info ***
For free Plug-ins, source and the Interface library for Visual C++ 6, .NET and now for Dev-C++
http://www.matrix1.demon.co.uk