The proposed patch accelerates ascii decoding in a particular case, when the alignment of the input data coincides with the alignment of data in PyASCIIObject. This is a common case on 32-bit platforms. I did not check whether the patch have any effect on 64-bit platforms.

Without patch:
$ ./python -m timeit -n 1000 -s 'enc = "ascii"; import codecs; d = codecs.getdecoder(enc); x = ("\u0020" * 1000000).encode(enc)' 'd(x)'
1000 loops, best of 3: 1.93 msec per loop
$ ./python -m timeit -n 10000 -s 'enc = "ascii"; import codecs; d = codecs.getdecoder(enc); x = ("\u0020" * 100000).encode(enc)' 'd(x)'
10000 loops, best of 3: 59.4 usec per loop

With patch:
$ ./python -m timeit -n 1000 -s 'enc = "ascii"; import codecs; d = codecs.getdecoder(enc); x = ("\u0020" * 1000000).encode(enc)' 'd(x)'
1000 loops, best of 3: 1.46 msec per loop
$ ./python -m timeit -n 10000 -s 'enc = "ascii"; import codecs; d = codecs.getdecoder(enc); x = ("\u0020" * 100000).encode(enc)' 'd(x)'
10000 loops, best of 3: 35.6 usec per loop