Just a brief update here.
While it’s still a work in progress, we have been working on making Discourse scale to even more categories and there is now an experimental site setting called lazy load categories groups available to opt into this behavior:
Those willing to live with rough edges are welcome to try it out, but we aren’t quite at the point where we’re looking for too much additional feedback yet.
Once we are, we’ll make create a topic in announcements here on meta to encourage more folks to try it out here and on their own site.