It’s a USB 3.0 Gigabit card, connected @1Gb, using the cabled supplied via the USB 3 port on my Surface 4, used before on a Fiber gigabit connection without issue.
Realtek USB GbE Family Controller
Antivirus besides the Windows Defender, plus Malwarebytes, both of which are disabled during testing complying with the test procedure.
CPU use is a healthy 1% during idle, and 30% during the test, so no infections or bottlenecks there.
Results are the same in safe mode, and in any speed test, far from 1Gb download (or better 940Mb) and very unstable.