We should create a "ThumbnailStorage" in storage.js that handles all our thumbnail saving/loading stuff. We definitely want to load thumbnails asynchronously. If this is too much for the scope of this bug we should at least provide an async API that acts sync in the background for now and file a follow-up bug.
Oh, of course we already load thumbnails asynchronously. What I wanted to say is we should load them in parallel. Maybe that's another bug though :)
This bug has already been fixed by the patch in bug 627239