bug#2530: 23/NS: redraws according to mouse-face are slow

[Eli Zaretskii <eliz@gnu.org>]

> From: Christian Kruse <cjk@defunct.ch>
> Date: Thu, 14 Jan 2016 22:39:20 +0100
> Cc: 2530@debbugs.gnu.org, Chong Yidong <cyd@stupidchicken.com>,
> 	Adrian Robert <adrian.b.robert@gmail.com>,
> 	Ian Eure <ian@digg.com>, Emacs-Devel devel <emacs-devel@gnu.org>
> 
> Compared to Linux (the Linux hardware is slower, a Notebook from 2011
> and the OS X notebook is a Macbook Pro Retina from 2014), the OS X
> version is *pretty* slow. While everything I do with Emacs is nearly
> instant when using Linux there is a notably delay when using Emacs with
> OS X.

That's normal: GNU/Linux is significantly more efficient than other
modern OSes.  Nothing related to Emacs, really.

> The worst example is Magit, which I already profiled because it is
> *so* slow: when using Linux `magit-status` shows up instantly. It takes
> about 1.5 seconds when using OS X (hold it, I am aware that this is not
> the place to discuss Magit performance, it is just an example :-) Every
> buffer with lots of lines (e.g. a notmuch buffer with 26k mails, my
> archive of the pg-hackers list) is lightning fast when using Linux, but
> takes round about 30 seconds when using OS X.

Sounds like you describe a situation that is file I/O extensive.  If
so, again, there's little wonder you see much faster operation on
GNU/Linux.  If you'd say the same about comparison with MS-Windows,
say, then it would be something worth investigating.

> Although I’m not sure that it is only the rendering engine (of course it
> could also be the elisp interpreter being slower) it occurs to me that
> it plays its part: especially redraw actions seem to be very slow. For
> example mu4e is unbearable slow when displaying maildirs with a lot of
> mails (e.g. the 26k mails maildir I mentioned above) but works fine for
> small mailboxes; and while the content of the maildir is loading, the
> buffer is flickering all the time as if it gets redrawn all the time.

The flickering you describe can only be triggered by
platform-independent parts of the display engine, so again, this isn't
OS X or NS specific, AFAIU.

Emacs comes with a trace-redisplay command (compiled only if you
configure Emacs --enable-testing='yes,glyphs'), so if someone wants to
test the hypothesis that such flickering is specific to NS, they could
run the same scenario on OS X and on another system, after invoking
trace-redisplay, and compare the outputs.  I'd expect them to be
identical (except for the addresses it prints).