Wait, do you mean do not use ZAZZ scroll and instead move a physical canvas over another image is less 'intense' computationally? Or is am I misunderstanding?
The zazz.scrol[] function is a thin wrapper around image.translate[], which updates every pixel of an image in-place.
Given a large source canvas/image that you want to scroll and a comparatively small window that you want to be visible at any given time, it is much less work to leave the source canvas static and copy out a window from a varying position than it is to scroll every pixel of the entire source canvas and then have Decker's compositor implicitly clip off the parts that are outside the card. See the example below.