From this pydev thread: http://mail.python.org/pipermail/python-dev/2012-July/120981.html

"BytesIO is actually missing an optimisation that is already used in
StringIO: the StringIO C implementation uses a fragment accumulator
internally, and collapses that into a single string object when
getvalue() is called. BytesIO is still using the old
"resize-the-buffer-as-you-go" strategy, and thus ends up repeatedly
reallocating the buffer as the data sequence grows incrementally.

It should be optimised to work the same way StringIO does (which is
effectively the same way that the monkeypatched version works)"