I'm using a 15" MacBook Pro running macOS Mojave and Windows 10 via Boot Camp. I bought a Thunderbolt to Gigabit Ethernet Adapter recently. After I connected it, I could select 1 Gbps Speed & Duplex when I started it on macOS, and Speedtest would show I have around 940 Mbps download speed. However if I started it in Windows, while the Internet connection status does show I have 1 Gb, Speedtest would show that I only have ~380-500 Mbps download speed.
I'm connecting it directly to my modem. The Properties of the adapter in Windows actually doesn't have the option to select 1 Gb, but only 10/100 Mbps Full/Half Duplex or Auto Negotiation. I have Auto on for this.
Some additional notes: I do have a 1 Gbps service plan from my ISP, and my family's regular Dell laptop could also achieve 900+ Mbps speed when connected.
Any thoughts on how do I get the same speed in Windows?