Well, all file I/O and a bunch of other things are in C, but all (or most of) that code probably isn't very CPU-intensive...
It should be possible to move the CPU-intensive parts (all the hashing & encryption parts) to C or Cython code. Alternatively, PyPy is working on removal of the GIL, but that might take years to finish.
But I'm not sure in how far Obnam currently uses non-sequential code anyway?