There seems to be a problem with the patch: when you store the getvalue() result somewhere (instead of discarding it), things get much slower:

$ ./python -m timeit -s "import io; n=2000; 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()"
1000 loops, best of 3: 913 usec per loop

$ ./python -m timeit -s "import io; n=2000; d=[b'a'*n,b'bb'*n,b'ccc'*n]*1000"  "s=io.BytesIO(); w=s.write"  "for x in d: w(x)"  "global y; y = s.getvalue()"
100 loops, best of 3: 4.67 msec per loop

This does not happen without the patch.