Why will Blender render the transparent area with shadow catcher on? One thing that puzzled me a lot is when I add shadow catcher to the ground floor, and render it, Blender will treat that transparent area as opaque one and render it as well, this causes extra render time waste, especially in the situation when batch rendering. 

Here is the image demo:



Does any one know how to avoid that extra render?

Or is this a Blender bug?
As far as I know Cycles still does not have adaptive sampling, so this is how it is for now. I believe this feature is always 'coming to Blender any time now' for a few years now. You can use border rendering to only render part of the frame.