The wired network on my iMac is uncomfortably slow. Google's speed test (just for one quick and easy metric) frequently dips down to about 1 Mbps for downloads. A separate laptop on the same network at the same time, but through WiFi, will consistently show 250 Mbps.
Sometimes connecting the iMac to WiFi seems to improve speeds a bit. Sometimes they improve on their own. In any case the iMac never sees more than half the speeds a laptop does.
This is a new problem immediately after upgrading to Big Sur (i.e., no other changes to the system or network). I'm therefore assuming this is something to do with macOS. But I am at a loss to even begin to know how to troubleshoot the problem.
What in macOS can I check or tweak or investigate to get any handle on what's even happening, much less how to fix it?