When the c extensions are not installed the pure Python BSON decoder performs unessecary memory copies. For example when decoding a string we first copy the bytes of the string (data[position:end]), and then decode those bytes to a Python string/unicode instance:

    return _utf_8_decode(data[position:end],
                         opts.unicode_decode_error_handler, True)[0], end + 1

https://github.com/mongodb/mongo-python-driver/blob/3.7.1/bson/__init__.py#L182

We can use a memoryview to avoid the need to create a temporary byte copy.

Credit to behackett for the idea.