Paraphrased from http://wiki.darcs.net/HowToHelp:

8. Try changing `FileName <FileName>`_ to store filenames as bytestrings
   rather than String. This should speed parsing somewhat, and reduce
   memory usage. We'd still need to convert to String for IO functions,
   but those are relatively isolated, so I think this'd still be a win
   (particularly as the filename starts out as a packed string in the
   patch files.

Sounds like something you'd need to benchmark.  Proceed with caution, as there
may be some compatibility/encodings stuff to contend with

Long since done by using AnchoredPath for all paths used in patches.