Can viewport playback be improved by moving elements to other layers? I am currently working on project that is made entirely of 2-D elements, images-as-planes.  I have the whole scene on one layer right now and the viewport playback speed has slowed down badly.  I have changed the fps to 24 and have checked "frame dropping" but the best it can do is 15 fps.  If i moved parts of the scene to different layers and then made them invisible would I be able to improve viewport playback speed? All of the elements are images-as-planes and are being loaded from an external hard drive; is there a way to bake an image-as-plane so it does not have to be loaded over and over?  

Would packing all the external data into the Blend file reduce the problem?

And finally, if 15 fps is the best I can do with the viewport, is there any problem with rendering the final project as 15 fps ?

Thank you!
Blender viewport performance is known to degrade with many objects, and frame rate can decrease significantly depending on scene complexity.

The rendering system is a bit archaic and doesn't currently take full advantage of high end hardware, it's planed to be overhauled in the near future, but unfortunately there is not much one can currently do to to overcome the underlying problem until then.

Where your images are loaded from (external hard drive, local disk, or packed in blend file) as little effect on viewport performance, except when reading them for the first time, before they are loaded into memory.

That being said, hiding objects that are not needed has a huge influence in viewport performance, you can either hide unneeded objects, move them to different layers, or even organized them into several distinct groups and use the Outliner to toggle viewport visibility en-mass. All those should have a positive impact in performance.

Additionally depending on what render engine you are using, (if you are in Blender Internal or Game Engine) you can switch to GLSL viewport shading mode which is said to have slightly better performance. N Properties Panel > Shading > GLSL

If you have an nVidia graphics card you can also try to turn on VBOs in the user preferences, which generally give quite a considerable performance bump in many cases. You can find it in User Preferences > System Tab > OpenGL Column > VBOs Checkbox.