Realize that your device is doing double duty, because it is drawing each frame twice; once for the left eye and again for the right. This extra work is being done for every pixel in your scene. By default, it also applies to objects not currently in view by the camera. As your projects increase in size and complexity, reducing the extra load on the processor is important for keeping the frame rate high enough for a comfortable VR experience. Currently, there are a few primary methods for reducing the processor's workload; removing objects that will never be in view, using single-sided GameObjects, and using occlusion culling.