A lot of headings in your Org-Mode file can make Emacs unusably slow when you collapse all of them.
My original solution was to avoid collapsing them with #+STARTUP: showeverything. Major problem with that approach is that you still can’t use collapsing because if you do, Emacs because again unusable.
Months ago I switched to nlinum and now there are no more performance issues, even on the largest file that I work on.