The reason why it lags on your computer is that Blender’s game engine it not made to create big games like Quake or something else, because the programmers never took peformance as number 1 when coding on the game engine.
Althought you can make your game run faster by doing a lot of things as keeping in mind the filesize of textures, sounds and another things, but things that really speed up your game are things like creating culling, level of detail script that show objects only when in close distance or increase detail when comming closer. Also splitting up your game in more then 1 blend file increase speed, no idea why but I think it has to do with the overall filesize of 1 blend containing a complete game.
The technic about sector culling was included in the game engine around version 2.1 or something, but this same technic can be archived with python.
Personally I hope someone will look into the game engine performace problem and update the engine on that part, maybe Ogre will do a significant chance in the game engine render performance, but I’m not 100%, we just have to wait a little while, but for so far I was the test build runs a way faster then the game engine does now.