> Anyone willing to try it out and report findings?  The manual needs
> to be fixed.

(setq gnus-secondary-select-methods 
      '((nnfolder "" 
          (mail-sources '((file :plugged t) ; get mail from system mailbox
                          (directory :path "~/Mail/incoming-nnfolder"
                                     :suffix ""
                                     :plugged t))))
        (nnml "" 
          (mail-sources '((directory :path "~/Mail/incoming-nnml"
                                     :suffix ""
                                     :plugged t))))))

Indeed, this works from what I can tell after some tests. 

But, why is it much slower than the approach in my previous mail? That
one needs around 1 second, this one now takes around 7 seconds (for 20
nnml and 30 nnfolder groups).

Does this different scope of 'mail-sources' perhaps introduce an
overhead that slows down processing of a group?