bug#17404: Scrolling is painfully slow

[Eli Zaretskii]

> Date: Tue, 06 May 2014 15:47:25 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 17404@debbugs.gnu.org
> 
> > Date: Tue, 06 May 2014 07:42:47 -0400
> > From: Richard Stallman <rms@gnu.org>
> > CC: 17404@debbugs.gnu.org
> > 
> > I should explain that I configured Emacs to visit that file in Rmail mode.
> > Try it that way.
> 
> I did.  I used "C-u M-x rmail RET foo2.mail RET" in "emacs -Q".  Does
> that reproduce the problem on your system?
> 
> What about "emacs -nw" -- do you see the same slowdown there?  Or is
> the problem limited to the GUI sessions?

Also, could you please try profiling Emacs and reporting the results?
Like this:

  M-x load-file RET simple.el RET
  C-u M-x rmail RET foo2.mail RET
  C-s GNU Emacs
  C-f
  M-x profiler-start RET RET

Now press and hold C-n for 5 seconds, then release C-n and type

  M-x profiler-report RET

In the buffer with the profile, please type "C-u RET" on every line
that has a "+" to its left, to completely expand the profile.  Then
please post the result.

Here's my profile:

  - command-execute                                                 159 100%
   - let                                                            159 100%
    - if                                                            159 100%
     - let                                                          159 100%
      - cond                                                        159 100%
       - prog1                                                      159 100%
        - call-interactively                                        159 100%
         - next-line                                                150  94%
          - if                                                      150  94%
           - if                                                     150  94%
            - condition-case                                        150  94%
             - line-move                                            150  94%
              - if                                                  150  94%
               - if                                                 150  94%
                - if                                                150  94%
                 - prog1                                            150  94%
                  - line-move-visual                                103  64%
                   - let                                            103  64%
                    - or                                             99  62%
                     - and                                           99  62%
                      - or                                           99  62%
                       - and                                         99  62%
                          >=                                         99  62%
                    - if                                              4   2%
                       let                                            4   2%
                    let                                              47  29%
         - list                                                       9   5%
          - read-extended-command                                     9   5%
           - let                                                      9   5%
            - unwind-protect                                          9   5%
             - progn                                                  9   5%
              - completing-read                                       9   5%
               - completing-read-default                              9   5%
                  read-from-minibuffer                                9   5%
  - ...                                                               0   0%
     Automatic GC                                                     0   0%

This seems to indicate that line-move and line-move-visual take most
of the time.  If you get a similar result, then please try replacing
these 2 functions with their code from May 2013, and see if that makes
the slowdown go away.