If you can’t see it, it’s not exist.

not Blender specific

I’ve come across many newbie’s project with quite heavy scene.
They would cry how slow their project would render.

One common mistake is that they place objects everywhere, like real world objects, even where the camera never sees.

So, if the object or its reflection, or shadow is not subjective in the camera’s field of view, it rather not exist at all – delete it! And make the set as small as possible.

Use ‘Scene’ to manage set

Blender 2.63a

When you have a set with lots of static objects, your outliner could be filled with those unidentified objects and sometimes gets in the way of selecting the right one.

You might join them to become one object, but it’ll be harder to move things around or assign new material later.

Better approach would be to create a new empty scene, place all those non-animate objects in this scene and group them. Rename the group to a meaningful name.

in the main scene, just add a ‘Group Instance’ with the group you just created.

Now, there’s only one object in the main scene, and you can still edit those individual objects by going to the ‘Set’ scene.