QTcpSocket is very slow with large packets when both peers are Windows machines

When using QTcpSocket to transmit data between Windows machines, performance drops dramatically when the packet size rises above some threshold. It's unclear if this is a problem in how we use the non-blocking API or if there is some very slow buffering in Windows itself, and the threshold seems to vary between machines (though it's hard to say what might cause that).

A test program indicates that data is passed into Windows very quickly (e.g. by printing debug output after each call to flush()), but the receiving peer gets it much more slowly.