StringIO:

$ ./python -m timeit -s "import io; n=2000; d=['a'*n,'bb'*n,'ccc'*n]*1000"  "s=io.StringIO(); w=s.write"  "for x in d: w(x)"  "global y; y = s.getvalue()"

-> Linux: 1000 loops, best of 3: 985 usec per loop
-> Windows: 100 loops, best of 3: 4.26 msec per loop

Unpatched BytesIO:

$ ./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()"

-> Linux: 100 loops, best of 3: 2.44 msec per loop
-> Windows: 10 loops, best of 3: 38.4 msec per loop

b''.join():

$ ./python -m timeit -s "import io; n=2000; d=[b'a'*n,b'bb'*n,b'ccc'*n]*1000"  "l = list(d)"  "global y; y = b''.join(l)"

-> Linux: 1000 loops, best of 3: 821 usec per loop
-> Windows: 100 loops, best of 3: 4.09 msec per loop

bytearray:

$ ./python -m timeit -s "import io; n=2000; d=[b'a'*n,b'bb'*n,b'ccc'*n]*1000"  "b = bytearray()" "for x in d: b += x" "global y; y = b"

-> Linux: 1000 loops, best of 3: 834 usec per loop
-> Windows: 10 loops, best of 3: 37.8 msec per loop