When bug 854795 is fixed, we'll have a new API for downsampling an image while it is being decoded, and we can use this to efficiently create thumbnails in the gallery app even when an image does not include an EXIF preview. (When images do include an EXIF preview, we should investigate whether it is worth finding that preview and basing the thumbnail on that. Or, if it is just as fast, maybe we can always use the full-size image).

I've got a work in progress patch in this branch https://github.com/davidflanagan/gaia/tree/bug949755 that modifies apps/gallery/js/MetadataParser.js to use -moz-samplesize I'm getting a 25% speedup on the time it takes to create preview images and thumbnails on my nexus4. (That device has such a high screen resolution that we can only use samplesize 2. The speed improvement might be better on devices with smaller screens and larger EXIF previews, but I only have the gecko patch build for my nexus4)

This got rolled into a patch in another bug. It has been fixed for a long time.