I guess you could something in the lines of $forum->topics->paginate(10) which would return in chunks of 10
Note that calling a relation without () will actually execute the query and get all relationship rows from the database and then call ->paginate() on the collection that is returned. So make sure you have the parentheses when you plan to build on the query.
Edit:
Same issue with using chunk().. it suggest to get every single row in your database table and then chunk them up in your application and only use the small part you need. It's like going to the store every time you need milk and buying every single milk package there is and carrying them home and then throwing all of them away and keeping 1 ☺