quad serializations take a long time


I hesitate to report this as a bug since I don't understand all that's going on in serialize, but following up on my report on the list a while ago <https://groups.google.com/forum/?hl=en-US#!msg/rdflib-dev/VgV9zNbk_cc/uVtOW5V74TkJ> all of the quad serializations take a lot of time. I did a crude time test on my graph of 75k statements, both parsing and serializing:

    >>> rdflib.__version__
    '4.1-dev'

    >>> def timeit():
    ...     start = time()
    ...     cg.parse('candidates.trig', format='trig')
    ...     elapsed = time() - start
    ...     print elapsed
    ... 
    >>> timeit()
    22.5815250874
    >>> len(cg)
    75840
    >>> def timeit():
    ...     start = time()
    ...     cg.serialize(format='trig')
    ...     elapsed = time() - start
    ...     print elapsed
    ... 
    >>> timeit()
    2746.05100608

As usual, I'm pretty far out of my depth here, but 45min seems long for a graph of this size.