Currently I have set up like this:
From router, SFP+ 10G going to my PC trough fiber.
From router, 10G RJ45 going to my NAS, to ASUS XG-C100C card.
Maximum speeds I get with this are 1G exactly, from PC to NAS.
When I connect NAS to 2.5G, I get 2.5G speeds exactly.
I checked my PC and NAS, and...