I pushed a change to optimize the ASCII decoder.

Attached bench.py script: microbenchmark on the ASCII decoder. My results follows.

Common platform:
Platform: Linux-4.1.5-200.fc22.x86_64-x86_64-with-fedora-22-Twenty_Two
Bits: int=32, long=64, long long=64, size_t=64, void*=64
Timer: time.perf_counter
CFLAGS: -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes
CPU model: Intel(R) Core(TM) i7-3520M CPU @ 2.90GHz
Python unicode implementation: PEP 393
Timer info: namespace(adjustable=False, implementation='clock_gettime(CLOCK_MONOTONIC)', monotonic=True, resolution=1e-09)

Platform of campaign before:
Timer precision: 57 ns
Date: 2015-09-21 23:48:00
SCM: hg revision=73867bf953e6 branch=default date="2015-09-21 22:40 +0200"
Python version: 3.6.0a0 (default:73867bf953e6, Sep 21 2015, 23:47:35) [GCC 5.1.1 20150618 (Red Hat 5.1.1-4)]

Platform of campaign after:
Timer precision: 55 ns
Date: 2015-09-21 23:52:55
Python version: 3.6.0a0 (default:bb0f55f1ec22+, Sep 21 2015, 23:52:43) [GCC 5.1.1 20150618 (Red Hat 5.1.1-4)]
SCM: hg revision=bb0f55f1ec22+ tag=tip branch=default date="2015-09-21 23:06 +0200"

------------------+-------------+---------------
ignore            |      before |          after
------------------+-------------+---------------
256 x 10**1 bytes |  314 us (*) | 5.25 us (-98%)
256 x 10**3 bytes | 31.9 ms (*) |  509 us (-98%)
256 x 10**2 bytes | 3.13 ms (*) | 50.2 us (-98%)
256 x 10**4 bytes |  315 ms (*) | 5.12 ms (-98%)
------------------+-------------+---------------
Total             |  351 ms (*) | 5.69 ms (-98%)
------------------+-------------+---------------

------------------+-------------+---------------
replace           |      before |          after
------------------+-------------+---------------
256 x 10**1 bytes |  352 us (*) | 6.32 us (-98%)
256 x 10**3 bytes | 35.2 ms (*) |  608 us (-98%)
256 x 10**2 bytes | 3.52 ms (*) | 60.9 us (-98%)
256 x 10**4 bytes |  354 ms (*) | 6.19 ms (-98%)
------------------+-------------+---------------
Total             |  393 ms (*) | 6.87 ms (-98%)
------------------+-------------+---------------

------------------+-------------+---------------
surrogateescape   |      before |          after
------------------+-------------+---------------
256 x 10**1 bytes |  369 us (*) | 5.92 us (-98%)
256 x 10**3 bytes | 36.8 ms (*) |  570 us (-98%)
256 x 10**2 bytes | 3.69 ms (*) | 56.9 us (-98%)
256 x 10**4 bytes |  371 ms (*) | 5.79 ms (-98%)
------------------+-------------+---------------
Total             |  412 ms (*) | 6.43 ms (-98%)
------------------+-------------+---------------

------------------+-------------+--------
backslashreplace  |      before |   after
------------------+-------------+--------
256 x 10**1 bytes |  357 us (*) |  361 us
256 x 10**3 bytes | 35.1 ms (*) | 36.1 ms
256 x 10**2 bytes | 3.52 ms (*) | 3.59 ms
256 x 10**4 bytes |  357 ms (*) |  365 ms
------------------+-------------+--------
Total             |  396 ms (*) |  405 ms
------------------+-------------+--------

-----------------+--------------+---------------
Summary          |       before |          after
-----------------+--------------+---------------
ignore           |   351 ms (*) | 5.69 ms (-98%)
replace          |   393 ms (*) | 6.87 ms (-98%)
surrogateescape  |   412 ms (*) | 6.43 ms (-98%)
backslashreplace |   396 ms (*) |         405 ms
-----------------+--------------+---------------
Total            | 1.55 sec (*) |  424 ms (-73%)
-----------------+--------------+---------------