To limit the lag on large worlds, the two most important techniques are culling and Level-Of-Detail (LOD).
Culling involves hiding polygons that won't appear on the screen anyway. DarkBASIC will automatically cull individual polygons that face away from the camera, so the only thing you can do to improve this is use HIDE OBJECT or EXCLUDE OBJECT ON when objects go out of the screen.
Level-Of-Detail is the technique of replacing a model with different versions of itself depending on how close the player is to it. When the player is really close to the model, a high-poly version is present. As they move farther away, the high-poly model is deleted or excluded and a lower-poly one is shown.
Oh, and just for randomness,
3d words.