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Actually, the slow speed in that specific circumstance has
nothing to do with the fact that it uses a string style
buffer, which should always be faster than what it used
before (an array of strings that was constantly appended to.)

The problem with that particular file is how the
gzip.readline function auto-optimizes it's read size.

if readsize > self.min_readsize:
  self.min_readsize = readsize

So, it optimizes it's read size to the length of the largest
line that it has seen so far.  The assumption is that
gzipped files are generally going to be a bunch of lines of
similar length, and not wildly differing length.