Currently the scheme used to divide the frame between machines only takes into account the relative power of the computers taking part in the render. This can have the effect of reducing performance when a scene has complexity only in one part of the screen. The image below shows this situation.
In this scene, where two computers with similar performance (cores in our case) render this scene, one machine will end up with a much higher workload since it will render most (if not all) of the planet with the othe machine only rendering the background.
Take account of the contents of the scene to ensure a balanced load between all machines on every frame. This will give maximum performance regardless of how the scene is composed.