We recently got a few Microsoft SP3's and some startech USB3 Ethernet adapter's - all looked good, however they really could not run at a true 1Gb/s - despite startech and MS claiming its fine.
Are there actually any USB 3 adapters out there that can sustain true 1Gb/s
Here's the thing about this, the firmware has tested the speeds at 1GB, but the overall speed will depend on the port you plug it into. If your computer is on the older side, your USB3 ports have had a lot of use, you might not be getting the full 1GB. That's unfortunately the way it is with USB ports, they do go bad over time and for ones on the motherboard, they cannot be replaced that easily. My recommendation would be to get a USB3.0 PCI/PCI-e card to plug into your computer and use that.