Ihor Radchenko writes:
You can try font-lock-profiler package (https://github.com/Lindydancer/font-lock-profiler)
This is useful, thank you.
It reports that most of the time is spent in org-do-latex-and-related, and some 20% in something related to the priority faces (despite the lack of priority cookies).
I doubt that those two account for the whole of the slowdown, but I'll be checking it out.
-- Sébastien Miquel