bug#21453: 25.0.50; When font size is temporarily increased Emacs is slow

[Eli Zaretskii]

> From: Mark Karpov <markkarpov@openmailbox.org>
> Date: Thu, 10 Sep 2015 15:56:46 +0600
> 
> When I press ‘C-x C-=’ to temporarily increase font size Emacs is
> getting considerably slower. For example, if I hold ‘C-n’ to move
> several lines down the movement is not smooth. I don't think that
> increasing of font size should normally result in such “effects”.

I cannot reproduce this in "emacs -Q" with today's master.  Do you
have any customizations related to scrolling?  This further input from
you:

> Also, when point reaches edge of screen Emacs instantly shows you next
> portion of text so point gets in the middle of the screen, instead of
> just showing you one more line.                            ^^^^^^^^^^
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

seems to indicate that you have customized scrolling, because by
default Emacs does not scroll just one line.  Perhaps you have
scroll-conservatively set to a large value; if so, I'd indeed expect
some slowdown when the font size is changed, because supporting
scroll-conservatively is sometimes costly, especially when the last
displayed line is only partially visible.

So please try this in "emacs -Q" first.

Thanks.