Slowdown with large headings in view

[Sebastian Wålinder]

Hello!

I have massive org-mode file with thousands of SRC blocks.

As the file grew, I started to get very poor performance, but setting 
`org-fold-core--ignore-modifications` to `t` solved it.

However, I still get very poor performance when a lot of folded headings are in 
view. It gets slower as headings with more text hidden inside its fold come 
into view.

I have tried running the profiler to find out what the issue is, but no matter 
how laggy Emacs gets, the profiler records practically no CPU samples or memory 
usage.

When the massive org buffer is in view and showing large folded headlines, any 
command run is very slow, even if the action is performed in another buffer 
entirely. It returns to normal when no window is showing the headings, as well 
as when I bring the folded headings out of view or unfold them.

I tried disabling font-lock, but it didn't do anything.

How can I profile this, or does anyone have a guess of what could be causing it?

Thank you!
Sebastian