I'm trying to modify minibb so (list topics, read and post but not admin functions) can appear as the contents of any block element inside an enclosing Content Management System (Robopages in my case, but any really). I have a working importer from phorum to minibb.

I have most of the "embedded minibb" system working too. But I seem to have a volume problem. I have a boat building forum (imported from ten years of phorum) with 20,000 posts. When I load that into minibb the topics display doesn't seem to be paging topics 50 at a time. Instead it loads all 20,000 topics, which takes a loooooooong time. Even on a test setup in localhost. I'm afraid to even try it on a real server.

"Save Page As" creates an HTML page whose wc output is 51977 273447 5378307
Youch.

Is there any way to make minibb display topics in bunches, as an ordered sequence? So the download times on 20,000 posts is not so long?

I'm not sure how this could be related to miniBB, in general. Surely miniBB has internal pagination mechanism. For example, on the start page of this forums you can see a bottom navigation which takes more than 170 pages. So it splits about 4,200 topics list into several pages.

This is actually your purpose to provide a valid output in your CMS.
I am not sure what did you did and how and so I can't give a proper recommendation on the exact code. But answering the question,

Of course, there is.