Consoles and interlaced video

[SonicIce]

I really dont understand interlacing. I mean take an Xbox for example. Say you're playing a 1080i game. The console is rendering the scene internally at 30fps. How is it possible for a console to run a game at either 720p or 1080i with nearly the same performance? Wouldn't 1080i require just as much processing as 1080p, but 1080i is only displaying half of the frame information each second while 1080p does all of it? im too tired to think, im probly making no sense