I wish I could help but I have no idea what's gong on either. It's either the mobo or card but I doubt it's the card because one card running at x8 after testing is pretty hard to miss. Got any other cards or mobos lying around for testing?
There might be an option in the BIOS for how fast the PCI Express bus is working? Not sure either. Disappointing to see that you have a $300 card and with limited performance
limited performance, no, at pcie 2.0 x8 you get something like <4% performance loss, that is nothing
@OP i would read through the motherboard manual very carefully if it says anything about the lanes being shared with something else
my guess is another device is sharing at least one lane and dropping it to x8