To improve the performance of a scene you can apply a Scene LOD which will allow you to specify when objects in the scene will stop being drawn based on the distance from the object.
To make a Scene LOD Right Mouse Click on a node in the Scenes tree and go to Create > Level of Detail.
In the properties of the SceneLOD you have created you can specify the in and out distances at which points the objects will be visible. In the example below the Objects under the SceneLOD node will stop being drawn when they are over 10 metres away from the user.
Once you have created the SceneLOD node you can place any objects you want to be effected by the scene LOD under the SceneLOD node.