Troubleshoot slow Big Sur wired network
The culprit here is the Cisco AnyConnect client. Apparently Cisco changed the way AnyConnect works for Big Sur (to get away from kernel extensions), and the new approach adds three &quot;filters&quot; that are visible in the Network panel.
The main filter shows as &quot;connected&quot; even when AnyConnect itself is not actually connected and indeed when the application isn't even running.
Removing the filters outright immediately restores network speeds back to normal. It also (unsurprisingly) breaks AnyConnect, which then tries to re-add the filters the next time it's run. That's going to be a problem for Future Me to solve.