Results of my micro benchmark. Use attached bench_stringio.py with benchmark.py:
https://bitbucket.org/haypo/misc/src/tip/python/benchmark.py

Command:
./python benchmark.py script bench_stringio.py

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Common platform:
CPU model: Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz
Python unicode implementation: PEP 393
Platform: Linux-3.4.4-4.fc16.x86_64-x86_64-with-fedora-16-Verne
Bits: int=32, long=64, long long=64, pointer=64
CFLAGS: -Wno-unused-result -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes

Platform of campaign pyaccu:
Date: 2012-08-10 04:24:53
SCM: hg revision=aaa68dce117e tag=tip branch=default date="2012-08-09 21:38 +0200"
Python version: 3.3.0b1 (default:aaa68dce117e, Aug 10 2012, 04:24:19) [GCC 4.6.3 20120306 (Red Hat 4.6.3-2)]

Platform of campaign writer:
Date: 2012-08-10 04:23:21
SCM: hg revision=aaa68dce117e+ tag=tip branch=default date="2012-08-09 21:38 +0200"
Python version: 3.3.0b1 (default:aaa68dce117e+, Aug 10 2012, 04:18:39) [GCC 4.6.3 20120306 (Red Hat 4.6.3-2)]

--------------------------------------+-------------+---------------
Tests                                 |      pyaccu |         writer
--------------------------------------+-------------+---------------
writer ascii                          | 30.4 ms (*) |        30.4 ms
writer reader ascii                   | 37.1 ms (*) |          37 ms
writer latin1                         | 31.5 ms (*) |        30.6 ms
writer reader latin1                  | 38.6 ms (*) |        37.4 ms
writer bmp                            | 31.8 ms (*) |  29.7 ms (-7%)
writer reader bmp                     | 40.8 ms (*) | 36.6 ms (-10%)
writer non-bmp                        | 33.4 ms (*) | 30.2 ms (-10%)
writer reader non-bmp                 | 40.9 ms (*) | 36.7 ms (-10%)
writer long lines ascii               | 7.96 ms (*) |  7.34 ms (-8%)
writer-reader long lines ascii        | 8.16 ms (*) |  7.39 ms (-9%)
writer long lines latin1              | 8.01 ms (*) |   7.4 ms (-8%)
writer-reader long lines latin1       | 8.05 ms (*) |   7.4 ms (-8%)
writer long lines bmp                 |   14 ms (*) | 9.42 ms (-33%)
writer-reader long lines bmp          | 14.2 ms (*) | 9.45 ms (-34%)
writer long lines non-bmp             | 13.9 ms (*) | 9.62 ms (-31%)
writer-reader long lines non-bmp      | 14.3 ms (*) | 9.63 ms (-32%)
writer very long lines ascii          | 7.96 ms (*) |  7.36 ms (-7%)
writer-reader very long lines ascii   | 8.05 ms (*) |  7.37 ms (-8%)
writer very long lines latin1         | 7.98 ms (*) |  7.33 ms (-8%)
writer-reader very long lines latin1  |    8 ms (*) |  7.39 ms (-8%)
writer very long lines bmp            | 14.1 ms (*) | 9.34 ms (-34%)
writer-reader very long lines bmp     | 14.2 ms (*) |  9.4 ms (-34%)
writer very long lines non-bmp        | 13.9 ms (*) |  9.5 ms (-32%)
writer-reader very long lines non-bmp |   14 ms (*) | 9.61 ms (-31%)
reader ascii                          | 6.48 ms (*) |        6.22 ms
reader latin1                         | 6.59 ms (*) |        6.57 ms
reader bmp                            | 7.22 ms (*) |         6.9 ms
reader non-bmp                        | 7.65 ms (*) |        7.31 ms
--------------------------------------+-------------+---------------
Total                                 |  489 ms (*) |  431 ms (-12%)
--------------------------------------+-------------+---------------