FYI, I found a workaround.
https://github.com/PyMySQL/PyMySQL/pull/409

_table = [chr(i) for i in range(128)] + [chr(i) for i in range(0xdc80, 0xdd00)]

def decode_surroundescape(s):
    return s.decode('latin1').translate(_table)

In [15]: data = b'\xff' * 1024 * 1024

In [16]: data.decode('ascii', 'surrogateescape') == decode_surroundescape(data)
Out[16]: True

In [17]: %timeit data.decode('ascii', 'surrogateescape')
1 loops, best of 3: 394 ms per loop

In [18]: %timeit decode_surroundescape(data)
10 loops, best of 3: 40 ms per loop