How is the performance when the terrain is diabled?

Hi,

I need to know which performance is better with the terrain or without the terrain.Help me with the terrain details like
1. what is it made up off?
2. Number of polygon used in a terrain and how can we reduce it.
3.Terrain data structure.
4.How can we limit terrian area.
5.How can we reduce polygon size.
6.Will this harm quality of the game.
7.How is the terrain rendered.Is the terrain loaded in memory and only texture is mapped at runtime.


I had come across a game there was no terrain instead it had a png in the bottom.Does this approach give a better performance.Can we have more number of dts file in this approach.

Thanks for the help.