So its running Gen2 x1, which is good. I was afraid that it might have downshifted to Gen1. Other threads point to your CPU being pegged, and I would tend to agree with that.
What direction are you running the streams in? In general, sending is much more efficient than receiving ("its better to give than to receive"). From your statement that ksoftirqd is pegged, I'm guessing you're receiving.
I'd first see what bandwidth you can send at with iperf when you run the test in reverse so this pi is sending. Then, to eliminate memory bw as a potential bottleneck, you could use sendfile. I don't think iperf ever supported sendfile (but its been years since I've used it). I'd suggest installing netperf on this pi, running netserver on its link partners, and running "netperf -tTCP_SENDFILE -H othermachine" to all 5 peers and see what happens.
What direction are you running the streams in? In general, sending is much more efficient than receiving ("its better to give than to receive"). From your statement that ksoftirqd is pegged, I'm guessing you're receiving.
I'd first see what bandwidth you can send at with iperf when you run the test in reverse so this pi is sending. Then, to eliminate memory bw as a potential bottleneck, you could use sendfile. I don't think iperf ever supported sendfile (but its been years since I've used it). I'd suggest installing netperf on this pi, running netserver on its link partners, and running "netperf -tTCP_SENDFILE -H othermachine" to all 5 peers and see what happens.