I am not see that BytesIO is slower than StringIO (both are about 30% slower than append/join).

$ ./python -m timeit -s "import io; d=[b'a'*10,b'bb'*5,b'ccc'*5]*1000"  "b=[]; w=b.append"  "for x in d: w(x)"  "b''.join(b)"
1000 loops, best of 3: 966 usec per loop

$ ./python -m timeit -s "import io; d=['a'*10,'bb'*5,'ccc'*5]*1000"  "b=[]; w=b.append"  "for x in d: w(x)"  "''.join(b)"
1000 loops, best of 3: 918 usec per loop

$ ./python -m timeit -s "import io; d=[b'a'*10,b'bb'*5,b'ccc'*5]*1000"  "b=io.BytesIO(); w=b.write"  "for x in d: w(x)"  "b.getvalue()"
1000 loops, best of 3: 1.22 msec per loop

$ ./python -m timeit -s "import io; d=['a'*10,'bb'*5,'ccc'*5]*1000"  "b=io.StringIO(); w=b.write"  "for x in d: w(x)"  "b.getvalue()"
1000 loops, best of 3: 1.24 msec per loop