I've installed a TX201 adaptor in my PC (the onboard is a 100/1000 port) thinking I'd take advantage of improved speed of my home network (TP-link Deco BE65, 2.5GbE ports) and internet connection (2000/2000 fibre connection), but doing a speed test now shows I'm now getting 450/450Mbps, whereas previously I was getting 950/700Mbps. This effect is visible by running speed tests one after the other and swapping the ethernet cable between the ports.

The TX201 is showing that the network is a 2.5GbE connection, so I'm not sure what's going on. Any suggestions?

I should note that I tried running a speed test on measurementlab and using the onboard ethernet I get retransmission rates <0.3%, but using the TX201 I get a retransmission rate >25%