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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?

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  • Did you check in Activity Monitor whether a process transfers a lot of data in the background?
    – nohillside ♦
    Commented Dec 9, 2020 at 12:38
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    Good point: This slowdown is true even with nothing running, so Activity Monitor shows no appreciable network or CPU activity.
    – VoteyDisciple
    Commented Dec 9, 2020 at 16:10

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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 "filters" that are visible in the Network panel.

The main filter shows as "connected" 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.

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