This continues discussion from bug 142590, where Paolo suggested that the following optimization could be implemented for Exif DateTime sorting:

if comment mtime < image mtime 
  read the exif data from the image and save it in the comment file
else
  read the exif data from the comment file
end

this way sorting by exif date is slow only the first time you visit a folder. This sort of exif data cache could be used in other parts of the code as well.

It would have to be written to be tolerant of read-only filesystems.

Perhaps the orientation tag could be in there too.

- Mike