Comment 2 for bug 1619362

dd'ing a sparse 4GB file still writes 4GB of zeros to my SD which takes about 20min (what is the magic to make it not do that that you mention above ?) while a 150MB file (as the pi2 image would likely be if we'd just map to the content size) takes below 1min ...

also when we release images we usually manually compress them using xz which takes a horrid amount of time for just compressing zeros ... (i understand cdimage will take care for this in future images for us though)

additionally you indeed force the user to have the amount of diskspace available when he wants to uncompress before writing the image.