[caf]

It could be the Nagle algorithm - setting the TCP_NODELAY socket option on the sender would be one way to test.

[graham_king_3]

Yes, thank you! Adding this line to the two-writes Python version (memg-slow.py on github) makes it about as fast as the single write version (memg.py on github):

sock.setsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)

It's an interaction between delayed ACKS and the Nagle algorithm, mentioned on the Nagle algorithm wikipedia page.

I'm learning a lot this week. Thanks again.