Cannot Get iperf close to 10GB

[iap128]

While troubleshooting slow transfer speeds, I ran an iperf3 test on my server and computer. What surprised me is that I'm not coming anywhere close to reaching 10gbps. As you can see from the screenshot, the closest I've ever gotten was 6gbps. I've tested two different PCIe network cards (one RJ45, one fiber) and an external Thunderbolt 2 NIC with pretty much the same or worse results. No matter what combination of setups I try, I cannot seem to break 6-6.5gbps. Any suggestions of what I should do? Here is what I'm using:

• Dell PowerEdge R710
• 10G SFP+ PCIe fiber card
• Sonnet Thunderbolt 2 10GB NIC
• Trendnet 10GB RJ45 PCIe NIC

I've tried bypassing my Netgear switch (direct from Trendnet to Sonnet) with no change. I've also tried using only PCIe cards with worse results. Unraid to Windows, Unraid to macOS, you name it, no change. The fastest setup I've tried is using the fiber card on the server, going through the switch, to my MacBook Pro using the Thunderbolt adapter. Worst config was using the RJ45 NIC in a Windows computer, reaching a max of 3gbps. I'm at a loss right now for what the issue might be and would appreciate any advice. 

[JorgeB]

Iperf performance is usually directly linked to the hardware used, if you can try with Mellanox ConnectX2/3 or Intel NICs, those are know to usually work well and give close to line speed, I use Mellanox on various Unraid servers and get around 9Gbits.

[jms2321]

You might try bumping the MTU size to 9K and seeing if that works.  I was able to do that and notice an improvement.  I am still fighting an issue with my 10Gbe on my network, having issues with performance from a server to my Unraid.  Oddly going the opposite direction seems to have no issue saturating my 10Gbe connection.  I am seeing a lot of retries on the slow direction.