That's an interesting point. The reason I am using the ImageAttributes as I am is for the spill image, in this case. I found that unless I tiled it in that fashion, it would fill only a portion of the area being drawn. I could stretch it to the full size, but that degraded quality too much. In a different shape (circular), the fact that the base image was smaller than the largest zoom images meant that it was filling only a portion of the circular area, in which case I was also using the ImageAttributes to fill the whole image.

It's interesting that you mention the speed difference, as I think that could account for some of the slowdown for the largest image. It would make sense that the ImageAttributes would have no impact as long as the tiling didn't actually happen, but would have an impact on the largest images where the tiling took place.