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(lpOverlapped & completion_func are always NULL), so there should
actually be no difference (well I'm now using recvmsg() instead of
recv(), and the overhead for 16 bit Winsock apps is definitely larger,
but I cannot see why that should slow everything down that much).
Perhaps I screwed up the blocking semantics of non-overlapped IO.

Have you generated the traces under exactly the same conditions?
Can you explain what exactly you did?

Martin