'tis a chrome/Firefox oddity. They're centred on Firefox.
Annoying, I know.
So, when purging a category with a few coughtwentythousandcough posts, everything crashes, including the VPS due to lack of resources. I'll be honest, didn't expect it to work, but you don't find out unless you try.
However even a category with 20-30 topics can take a while to actually purge. Might be worth adding a similar thing to Restarting NodeBB along the lines of "Purging".
EDIT: Don't know why I put this in bug reports, probably more a feature request.
Made some changes here to speed up the category purge.
I'm not sure how much it will help though since src/posts/delete.js has still quite a bit of work to do to prune each post.