The load test utility had maxed out the compute resource that it had available, due to internal limitations within iperf itself. It’s a shame that this utility is not multi-threaded, because I think we could have a much greater result of improvement on this system.
Monitor the utilities that you’re using to do load testing, because limitations like this might skew your results!
Everything eventually hits a bottleneck. In David’s case, the bottleneck was in the testing tool itself.