Request: Garbage Mask or ProcessingBox for speed gain

Hi, 
many thanks for your great work so far!
Do you think it would be possible to add a Garbage Mask so only the white, active part will be processed, this coulld be even a mask per clip and not per frame or generated from the alphahint? 
In the simplest version just a box containg the alphahint with little space around. 
Like this the ressources would not be wasted on cut away parts of the frame. 