Brocade IP Ethernet issues

Problem Description: Basically, I cannot get four separate (with identical HW and OS) servers with 40GbE Intel XL710 NICs to sustain 30-40Gbits/sec using perf or iperf3 with or without multi-threading. These servers are quite modern with 20 logical cores and the latest PCI backplane technology (5GBytes/sec per slot that is associated with a particular CPU socket). We see many retransmits during our iperf3 tests, but no L1 errors on the switch links. The retransmits are a huge clue, I suppose. All connection between servers during testing are on the same switch and same subnet. Hardware we are working with: Hardware Vendor:ICX7750-48C BROCADE ICX 7750 WITH 48 1/20 GBE RJ-45P Hardware Vendor:ICX7750-26Q Brocade ICX 7750 with 26 40 GBE Qsfp=Por