follow-mode: extremely slow in combination with org-mode

[Gerald Wildgruber <wildgruber@tu-berlin.de>]

Hi,

I have got a problem with enormous lag while entering text in buffers
with emacs follow-mode enabled.

I'm using emacs (git checkout v. 27.0.50) and Org mode (git checkout
release_9.1.13-760-g8def68).

My work is solely text-based, using org-mode. A typical setup is to use
a maximized or full screen emacs frame split into five windows
positioned vertically one next to the other on a 40" 4k display. All
five windows display one and the same file, that is opened using emacs
follow-mode, so that every window is displaying a portion of the same
file in a continuous manner (simultaneously displaying up to 45kb of
text in one frame, an entire paper).

Unfortunately, with this setup there is terrible lag with every single
key input (on a very fast quad core machine); every key stroke produces
a 100% processor load.

If I deactivate follow-mode, the problem disappears. It also gets better
if I enable text-mode instead of org-mode.

I then used the elisp profiler (M-x profiler-start/report) to find out
which function uses most cpu time while editing text in said setup. Here
is the result:

Collapsed, the report looks like that:

+ command-execute                                                8789  47%
+ follow-post-command-hook                                       7755  41%
+ ...                                                            1976  10%
+ redisplay_internal (C function)                                 104   0%
+ yas--post-command-handler                                        40   0%
+ timer-event-handler                                              20   0%
  tooltip-hide                                                      7   0%


And somewhat expanded:

- follow-post-command-hook                                       7755  41%
 - follow-adjust-window                                          7755  41%
  - follow-windows-start-end                                     7732  41%
   - follow-calc-win-end                                         7732  41%
    + pos-visible-in-window-p                                      25   0%
    + posn-at-x-y                                                   7   0%
    + window-inside-pixel-edges                                     3   0%
  + follow-all-followers                                            4   0%
    follow-avoid-tail-recenter                                      3   0%

If I understand correctly "follow-calc-win-end" would be the function
that uses most of cpu time.

I then did additional profiling with the elp library, adding relevant
functions under "Command-execute" and "follow-post-command-hook" to
elp-function-list, edited text, and then did M-x elp-results. It showed
again that "follow-calc-win-end" by far used most of cpu time.

Anyone got an idea what's going on here and how to debug that? Are there
possible optimizations with this situation? Or is this "normal",
expected behavior, simply due to the number of windows and text
displayed?

Thanks

Gerald.

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