Some thoughts from reading about throughput performance on the RxS devices.
Test throughput _though_the device and don't run the test with the R4S as an endpoint.
Default settings might have IRQs, queues and other processes on the same core or similar.
Use top or whatever appropriate tool to watch per CPU load during the tests.
Look at assigning processes to specific cores or try irq-balance.
More info on the device page: