Various actions on megatopics would cause timeouts on my forum: 502 errors when splitting topic, moving or renaming categories, including renaming a category.
I’ve tested these actions on a lot of categories, the action durations correlated with the number of topics/posts from the categories
I had to rename a few categories by updating the database fields from the command line instead of editing the category settings inside Discourse’s interface.
I have been able to split the megatopics after a lot of tries, but not the largest one though I didn’t try since my migration 1 month ago.