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The tests are running on multicore-machines, hopefully QT distributes them on the processors. (with WinAPI, i know there is a function called SetProcessorAffinity() or something like that).
Qt has nothing to do with that. It assumes the operating system "knows better", so threads might be scheduled to the same processing unit. You should be able to verify that using system monitoring tools.
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What makes me wonder, is the huge difference between the C WinAPI, C# and C with QT in throughputs. The receiver applications are all done in the same way, one thread for GUI, one only for receive. C, C# allow 900-950MBit/s with moderate loss, Java is slightly more worse, but C with QT.
You may use a profiler to find the bottleneck. Seems that you are not reading data fast enough, but why - that I don't know.