Hey,
I have a simple scene that contains basically a box and a sphere. The world has a HDRI environment texture. The box has a basic diffuse shader applied and the sphere has a relatively simple glossy based shader. The scene is part of an animation sequence where the sphere is the only rendered object, the box and environment are not visible to the camera they just provide a reflection. The entire scene is transparent apart from the sphere. I am using Cycles also.
The frames render perfectly, however it takes about an hour or so to render each frame. I have it setup to render 32x32 tiles which does help with the speed but almost the entire hour of rendering is consumed by blender rendering the completely empty and transparent tiles. The tiles that contain the sphere are complete in a few minutes, so my rendering times are increased by 15-20 times! Considering that I have more than 300 frames to render this is going to add days of rendering to the project.
What I am wondering is why does blender spend so much time on completely useless areas of the frame? Even if the sphere is on the first tiles rendered it appears as it should so it is not a case of the environment needing to be rendered in the background, it just seems like blender should be smart enough to know that tiles that contain no visible objects at all can be skipped entirely.
Is this normal? or am I maybe doing something wrong in my settings?
Thanks. 
P.S I did try to post this thread earlier but it doesn’t seem to have shown up…