Be an artist and make it nice ;).
Now you got the point of a game. It is always a balance between “looking good” and speed. For a game speed rules. Similar to movies: fake, fake, fake as much as you can.
Everything of a scene is hold in memory, even if you suspend it.
The speed of rendering depends on how many faces are seen by the camera (inside the camera frustum).
There is some optimization.
e.g. frustum culling: Objects that are completely outside of the frustum culling are not parsed for rendering (that means not every single face is checked if it should be rendered)
Faces have no backside. This reduces the number of rendered faces by at least 50%. But you need to close your objects.
The speed of geometry transformation depends on the number of vertices. Remember each single vertex has a position in space. This must be recalculated at each single frame.
Also the number of vertex groups of an mesh has influence on the speed.
The speed of the logic depends on the number of objects, number and type of logic bricks, number of triggered controllers/actuators.
The speed of the pyhsics depends on the number of faces and number of objects with and without physics enabled. There is no frustum culling, so physics works even if you do not see it.
As you see there is a lot that influences speed.
as more you have at one time as more it eats speed and memory at this time.