There can be a number of reasons why follow-mode is slow. The main problem is that is it's time consuming to recompute the window state, and even more time consuming when aligning the windows. For simple commands like point movement, follow-mode assumes that the content of the buffers hasn't changed and retain the existing layout, whereas for other commands (including self-insert-command), everything is recomputed and realigned.
However, I use six side by side windows, and a small font, giving me a total of almost 800 consecutive lines, and it typically runs fine without any noticeable lag.
There are a number of factors that can have a negative impact on follow-mode:
- It run slower on a 32 bit binary than on a 64 bit binary (at least under Windows). (For this reason, I used Emacs 22 at work for many years, until prebuilt 64 bit binaries of modern Emacs versions were available.)
- Follow-mode is has problems when the width of the windows differ, especially if you have long lines that spill over from one window to another. I've written a support package to set up side-by-side windows in a pixel-perfect way, https://github.com/Lindydancer/multicolumn
-- please try if and see if follow-mode runs more smoothly.
One thing that I have had on my wish-list for a very long time (20+ years) is that normal typing should use and update the cache, to speed things up. Of course, this only work as long as the editing doesn't change the layout, like when inserting a newline or make a line so long that it wraps. Unfortunately, I don't think that I will have time for it anytime soon, but I'm happy to share my ideas if someone else is willing to make a try.
What made me curious that you said that the slow-down only occurs with org-mode. My guess is that invisible text makes it harder for Emacs to calculate window layout properties.
Anders Lindgren (I wrote follow-mode when I was a student, in the mid 1990:s)