As I said earlier on the mailing list, I suspect (without looking into it) that the cause of the I/O performance slowdown relative to C++ is something related to buffering—perhaps the I/O is not being buffered, or the buffer isn't functioning right. This would be consistent with the serialization-based I/O leading to larger slowdowns, because there would be more calls to write(2) then. If so, then this should be fixable.

I'm pleasantly surprised to see the object mode slightly faster than C++.