Since propagation of information is limited to one cell per step, it's possible to speed this up by splitting the image into smaller subimages and solving those first, and then solve the borders between such subimages.
This technique can be used to solve multiple steps as well, the only change is that the border between subimages becomes thicker.
This technique can be used to solve multiple steps as well, the only change is that the border between subimages becomes thicker.