well, that's what I was hinting at by the fast 1/8 reduction, so yes you can speed up the process by "decoding" a smaller version of the jpeg image.
This is just an optimization to add later though. You might be able to quickly scale your jpeg down, but you can't skip the rest of the transforms, because you still need to get deterministic fingerprint of the image. And in the article's use case, the file size is so small that you shouldn't be dominated by decoding time (that test jpeg from the site decodes in 12ms on my laptop with libjpeg at full size).
