Ethernet Speed Capped by macOS over Thunderbolt?

<< This is clearly a macOS and not a hardware issue >>

Not necessarily. There are other limiting factors you do not appear to have taken into account.

Also, MacOS happily does 10Gigabit Ethernet in iMac Pro, certain Mac mini, and Mac Pro 2019, and Mac Pro silver tower with aftermarket adapters.

One bizarre thing that happens is that when running this fast with default 1500 byte frames, you can run out of frame numbers before the acknowledges start to come back, forcing a delay while waiting for an acknowledge. So use of Jumbo frames is important. Support for sizes up to about 8000 are common, but not universal. Support for higher sizes varies by each device that touches the packets, so whether a certain device supports jumbo frames and what maximum size needs to be tested.

My Mac Pro silver tower frame size was set to 8163 after an afternoon of testing, and a week of re-cabling because the Routers I was using as Switches could Not handle Jumbo frames at all.

There have been a number of reports that suggest M1 Macs that accept ThunderBolt Power may thereafter have limited abilities to do other stuff on the same ThunderBolt port.

An experiment might be to start your M1 Mac with the power adapter connected BEFORE you connect the display, then test again.

Based on that, I think you may be suffering from a problem between MacOS and the RealTek 8139 chip.

That chip is used by almost all ThunderBolt-to-Ethernet adapters, even the Belkin one Apple sells in the store. Users have reported that it disconnects under heavy loads, but frankly, I expect none had ever measured it more than just once, and since it was as fast as their Internet connection, thought nothing of it.

You point out that it is also exceptionally SLOW from day 1.

There are reports that the latest update to Monetary 12.0.1 fixes the RTL 8139 driver issues (but that may only be a fix for disconnecting).

If you are inclined to test that version, readers would LOVE to hear your results!

I am also having this issue. I have thunderbolt dock that has realtek 8153 ethernet port and speed is capped it 650/750 mbps. I also have a usb realtek 8153 dongle and when i connect this directly to m1 max tb port i am getting max speed.

I called apple and talked withe senior support adviser and he told me that it is a problem with thunderbolt dock and apple does not troubleshoot third party products. The same docks gets the max speed in windows and linux. There should be some problem in apple supplied drivers and this is not a hardware issue.

And also when i have the dock connected and restart the m1 max the magsafe turns amber and it stays like that. I need to disconnect and reconnect the magsafe to fix problem.

Hope these are all monterey problems and apple will fix it soon in upcoming next versions.

You are right the answer what apple gave to me is not responsible answer. Even i was shocked by his answer. realtek ethernet chipset is a popular one. It is everywhere. It is a shame that big company like apple does not make it right and blaming third party manufacturers whereas they work fine with windows/linux. Even yesterday i tried to create monterey usb bootable disk with sandisk cruzer usb disk it was taking more than 4 hours and only 60% complete. I tried with intenso usb disk and it was quick. The san disk cruzer works well with windows/linux.

My bad, I got the chip wrong.

The 8139 came out in 1999 and only goes up to 10/100. So it was never used in the Gigabit-capable adapters available today.