gday mate!

give us a screenshot of your level, because it sounds to me like you are either using too hi-poly models, your textures are too big, you have too many entities on screen, or many other things. the community and i wont be able to give you specific help until we know what you are trying to run in realtime!

here are some tips:
use LoD -- models switch to lower-poly/res versions when they are further from the screen;
use less models -- instead of having a whole bunch of rock entities near each other, have one entity that contains all those rocks. less entities is generally better;
custom culling -- in your script, try to make small and less-noticeable entities (like grass) fade out to invisible at a fairly close distance, and try to make your own code that tells your objects to turn invisible or even remove themselves (they can be put back in when they are needed with more code) when there is no way for the player to see them from their position (for example, objects that are indoors while the player is outdoors, or objects behind a large wall);

plenty of other 3DGS users have better tips than i do, im sure. there is a thread under "game design" i think, that has hundreds of tips on improving framerate.

hope this helps!

show us a scene that runs really slow!

julz