Recently I switched to an Awow AZ51 micro PC for OPNSense. This device has two 2.5Gb ports, and most users report having to change settings to get full throughput. When I set it up initially, I got limited throughput, and found I needed to set the Speed and Duplex setting for each interface to 1000BaseT, and got full speed.
Today, to troubleshoot a different issue, I had to revert to an older saved configuration, which did not have the Speed and Duplex setting. After a reboot, imagine my surprise when I got full speed without changing the Speed and Duplex setting from "Default."
I'm happy about it, but I don't understand it. Could the hardware interface have remembered the setting?
I suspect layer 0 gremlins. Did you buy a decent CAT6A cable?
Depending on what's on the other side of the cable autonegotiation may fail or be hit and miss.
It's best to control it on the FW side in this case.