Assuming the image has a small thumbnail embedded in the EXIF data, maybe it would be even faster to use that resource (already included with the image), scaled to fill the image space and replaced when the image loads.
Essentially the same effect, with some back-end code. I suppose we'd need a benchmark test to find the real numbers. The main advantage would be sticking with existing file types already in use.
JPEG already supports multiple images per file. If you add EXIF data with manifest information with web rendering hints at the head, your thumbnail in the next frame, and your full-sized image after, you'd be on the track to awesomesauce.
(I'm sure someone has already implemented something better than this, but I am lazy.)
In EXIF, whether applied to JPEG or TIFF, the header comes first, then the thumbnail if present, then the primary image. Exactly as you'd want (which is more than can be said for much of EXIF, e.g. that it lacks time zone encodings).
Essentially the same effect, with some back-end code. I suppose we'd need a benchmark test to find the real numbers. The main advantage would be sticking with existing file types already in use.