Here is a preliminary version of the patch. I am not sure that it is fully correct.

Microbenchmark results:

$ ./python -m timeit -s "import io; n=100; d=['a'*n,'bb'*n,'ccc'*n]*10000"  "s=io.StringIO(); w=s.write"  "for x in d: w(x)"  "s.getvalue()"
10 loops, best of 3: 25.5 msec per loop
$ ./python -m timeit -s "import io; n=100; d=[b'a'*n,b'bb'*n,b'ccc'*n]*10000"  "s=io.BytesIO(); w=s.write"  "for x in d: w(x)"  "s.getvalue()"
10 loops, best of 3: 39.9 msec per loop
$ ./python-patched -m timeit -s "import io; n=100; d=[b'a'*n,b'bb'*n,b'ccc'*n]*10000"  "s=io.BytesIO(); w=s.write"  "for x in d: w(x)"  "s.getvalue()"
10 loops, best of 3: 26.1 msec per loop

$ ./python -m timeit -s "import io; n=1000; d=['a'*n,'bb'*n,'ccc'*n]*1000"  "s=io.StringIO(); w=s.write"  "for x in d: w(x)"  "s.getvalue()"
100 loops, best of 3: 12.1 msec per loop
$ ./python -m timeit -s "import io; n=1000; d=[b'a'*n,b'bb'*n,b'ccc'*n]*1000"  "s=io.BytesIO(); w=s.write"  "for x in d: w(x)"  "s.getvalue()"
10 loops, best of 3: 26.5 msec per loop
$ ./python-patched -m timeit -s "import io; n=1000; d=[b'a'*n,b'bb'*n,b'ccc'*n]*1000"  "s=io.BytesIO(); w=s.write"  "for x in d: w(x)"  "s.getvalue()"
100 loops, best of 3: 13.6 msec per loop