bug#18522: 24.4.50; mapcar is very slow

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: pmlists@free.fr,  18522@debbugs.gnu.org
> Date: Sat, 20 Feb 2016 09:46:23 +1100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> >   (let ((gc-cons-threshold most-positive-fixnum))
> >     (benchmark-run 1
> >       (dotimes (i 10000)
> >     (parse-time-string "Fri, 13 Feb 2015 14:40:02 +0000"))))
> >
> > FWIW, this takes 1.2 sec in a fresh "emacs -Q", and 2.0 sec in a
> > full-blown session running for 5 days.  That's clearly different from
> > the data presented by Peter (but I don't use Gnus).
> 
> Well, it's almost twice as slow in a five day old Emacs...  doesn't that
> show that something weird is going on somewhere?

2.0 divided by 1.2 is 1.7, not 2.  And Peter's data shows a 3.7-fold
slowdown after only 3 days.  So the difference is IMO striking.

Nevertheless, profiling parse-time-string on a more fine-grained level
seems the way to go.