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When using `org-publish-project` I noticed that generating a sitemap
sorted (anti-)chronologically is very slow. It turns out that the
slowness is due to the sorting of sitemap entries, which calls
`org-publish-find-date` during the comparison. But
`org-publish-find-date` is not cached therefore it is called over and
over for each file during the sorting process.
By contrast, `org-publish-find-title` is cached and sorting
alphabetically is faster.

To test this assumption, I've modified `ox-publish.el` to cache
   `org-publish-find-date`, too, obtaining a substantial speed-up:

   (defun org-publish-find-date (file)
     (or
      (org-publish-cache-get-file-property file :date nil t)
      (let ((date (org-publish-find-date-uncached file)))
        (org-publish-cache-set-file-property file :date date)
        date)))

   (defun org-publish-find-date-uncached (file)
     "Find the date of FILE in project.
   This function assumes FILE is either a directory or an Org file.
   If FILE is an Org file and provides a DATE keyword use it.  In
   any other case use the file system's modification time.  Return
   time in `current-time' format."
   ...

Is there a reason why we should not cache the date of a file, in the
same way as we cache its title?

Thanks,
Lorenzo

Emacs  : GNU Emacs 25.1.1 (x86_64-unknown-linux-gnu, GTK+ Version 2.24.23)
 of 2016-11-01
Package: Org-mode version 8.2.10 (release_8.2.10 @ /opt/emacs/lisp/org/)