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.