Slow ethernet under windows, normal under Debian

My ethernet speeds are lower than expected on this computer. There are two ethernet adapters on this motherboard, one 2.5Gbps and a gigabit adapter. They have different manufacturers and run completely different drivers.

Internet speed tests and LAN throughput tests (with iperf under WSL) hover around ~550Mbps. A Macbook Pro, with the same ethernet cable on the same port on the gateway, hit 780Mbps down over the internet.

So to eliminate a hardware issue, I installed Debian 10.9 on a spare disk today - the throughput over LAN magically jumped to 980-990Mbit/s. This is on the same computer, with the same cable, on the same port on either side - just reboot into a different OS and the speed doubles.

I'm at a loss here - I've disabled LSO for ipv4/6, energy efficient ethernet, and a host of other things. This is under W10 ver 2009, 19042.985.

No luck. I'm investigating tcp window sizes at the moment and I'll get back to you on that. Interestingly, it looks like only download is affected - upload over LAN seems to be fine from this computer.

that's right, this is wired. Both ethernet adapters are integrated - I have one called "Intel(R) I211 Gigabit Network Connection" and another called "Realtek PCIe 2.5GbE Family Controller". For reference, the board is an ASUS Crosshair Hero VIII.

My router is only okay, so I don't get great speeds over wireless. Most of my devices are wired. I'm busy with work at the moment but I'll test wifi as well later today and get back to you on the exact speeds I get for up and down on each adapter.