Thread by @ErrataRob

So I discovered that the 'ping' latency in Speedtest.net is a lie, at least for DOCSIS cable modems. It says 10ms, but it's closer to 40ms for most people. That's the minimum latency added by cable modem technology.

In the above speed test, I opened Wireshark to capture the session, then looked at the "TCP round-trip time". As you can see, I'm getting around 25ms round-trip. This is DOCSIS 3.1 w/ AQM. DOCIS 3.0 was giving me about 45ms to the same server.

This is a known issue of DOCSIS cable-modem technology, dealing with the fact that multiple customers can't transmit at the same time. When the cable is lightly utilized, it adds 10ms latency. When heavily loaded, it can go up to 100ms.

They are working on a low-latency option for games and voice that need it, but it's still not deployed yet.

The upshot: cable-modems add 10ms to whatever other latency you might have at minimum, and up to 100ms when heavily loaded.

By "I discovered" at the top of the thread, I mean, "I discovered what everyone else who works in that industry already knows", not "I discovered something new that nobody knows". I hadn't been paying attention to this and I should've.

I also discovered that Comcast locks onto the MAC address of your router for 24 hours. So when I got my new modem, I first tested with a USB Ethernet, which was the limiting factor (USB sucks). So I had to retest using Thunderbolt adapter.

But the new Thunderbolt adapter wouldn't work. Testing with masscan spoofing the old MAC address show the problem was their router would only respond to the MAC address of the USB adapter (masscan allows spoofing of MAC addresses).

Changing the Ethernet MAC address on macOS is easy -- if the driver supports it. Unfortunately, this Thunderbolt adapter didn't support it.

But I waited 24 hours for Comcast to forget the old adapter, then the new one worked.