The `Parallel` version is slightly slower (12s to 13s on Brown corpus benchmark) compared to the 30 lines here. Is the principle here more towards aiming for better maintenance, in the cost of slightly lower performance or should we aim for the best performance?
However, the disadvantage of using the 30 lines here is that it can't accept `raw_documents` as iterator, it only accepts list. So I guess this slightly lower performance is acceptable.