In your general prefs:

Make sure render to separate process is checked and up your number of threads to double whatever the number of cores you have.  If you have 4 cores, go to 8 threads.

Lower the number of undo levels.  It defaults to 100.  The more undo levels you have, the larger the cache windows seems to hold onto.

Do not use the render cache to save renders.  Lower it to 1.  If you keep renders cached this is another resource eater.

If things are really slowing down, take a look at texture sizes.  Don't use high res textures on background and midground figures and props.  You can make your own mid and low res textures by taking the textures into an image editor, lowering the size of texture and saving under a new name.

Background and midground figures don't need all the bump and spec maps. 

If you are using full dressed figures, hide as many body parts on the base figures and clothing as you can.

If the camera can't see it, it won't render it so it doesn't need to be in the scene.  Unless you are using a figure to create a true reflection.  This is probably the only exception I know of.

As stated above, use only the morphs you need.  If you need to add all the face morphs to get the face, do that first.  Then save the face morph.  Create a new scene.  Load a base figure without all the morphs added and inject the saved face morph.  This keeps the cr2 lean.  Same for body morphs.