New Logic Board & Gigabit Ethernet Down to 2 Mbps

I have a 2020 iMac 5K. I had a warranty repair done due to persistent power failure problems. That was in December. Apple warrantied the repair and replaced the logic board. Turned out, they installed the wrong logic board - putting in one with the incorrect video chip from my original, custom order. So they replaced the logic board again yesterday. I picked it up today, and brought it home.

First thing I noticed is that I couldn't restore from my Time Machine backup. Then I noticed the logic board (with soldered on-board SSD) they used was running an older version of macOS -- Catalina 10.15.7. Which is extremely odd unless they're using a refurbished board, which they aren't supposed to be doing. But either way, Time Machine backups aren't backwards-compatible, so I decided to download Monterey and upgrade before restoring from my backup.

But the download is running at a glacial pace.

Speedtest results are pulling 2 Mbps down, and about 95 Mbps up. I have a direct connection via cat 6 Ethernet to a 1 Gbps fiber optic line straight to the house. Every other device in the house (an older Mac Pro, multiple Windows laptops, and countless smart TV and mobile devices) all work fine. Just in case, I rebooted everything, including the router. It's still not running more than 2-4 Mbps. If I turn on wireless and run that instead, this machine will get up to 12 Mbps.

I called our fiber company. There are no service outages or degradation in this area.

Is it possible there's something wrong with this logic board, too? Or something else I haven't thought of?