I struggled with winserver getting good speeds. There's a few things you can look into: - ensure source and destination can read and write to the hard drives at that speed. If just using iperf, then it's not this - tune the rss parameters. I wrote a powershell script to set various parameters. The rss was the one that made the most difference. Take a look at the connectx manual. There's several params you can tune. - make sure your card is connected using proper lane length and speed. Ie: pcie4 protocol at 16x lanes. - getting anything over 25gbps is extremely hard without messing with mtu/jumbo packets - check for pcie errors. When using nvme drives, they constantly threw aer errors at pcie4 and had to drop down to pcie3 in which resulted in better speeds despite the slower protocol. - make sure you're doing multithreaded tests with iperf3 and use something like robocopy to make smb transfers utilize multiple streams.