MacBook Pro M4 Ethernet speed reverts to 100 Mbps with USB-C adapter

IMHO, I'd suspect the adapter before anything else.

Have you tried shutting down your MBPro (full shut down), disconnect all peripherals including the TP Link adapter, restart. Then make sure WiFi is turned off and do another full shutdown. Restart again, then once fully restarted, connect your TP Link adapter and see what the performance is.

I use a Belkin adapter (INC012) with my M4Pro MBPro and have never had a problem with it.

It would also help to know what you are connecting to (e.g. your router) ... it's possible that the ethernet ports on the router may only support 100Mbps. The cable(s) may even be a limiting factor - older Cat5 and earlier cables are limited to 100Mbps; you need at least Cat 5e for 1Gbps support.

That suggests the connection is stuck at or below 100 M bits/sec.

There are two common reasons for that:

• your cables do not have four full pairs of wires, correctly terminated.

Look through the clear end of the connector, and make certain all 8 wires are present.

Ethernet at or below 100 M bits/sec uses baseband signaling, and only requires two pairs. Speeds above that REQUIRE all four pairs, because 1000 and above uses a modulated signal that requires the full spectrum of all four pairs in the cable.

this shows one of the two most common ways of wiring a cable-end.

Also, the newer Mac Studio models that feature 10G Ethernet DO NOT support baseband signaling at all -- they can not run slower than 1000.

• the device at the other end of the cable must be capable of 1000 M bits/sec. if it is not, you can't go any faster.

consider this:

Actual Speed:

The good way to check the actual connection speed USED to be Network Utility, But in Catalina and later, Apple has deprecated Network Utility and now you have to use a Terminal command to see your actual connection speed. First, you need to know what en number the link is. then you use a command like this one, substituting the actual en number.

my main Ethernet connection uses BSD name en2 (as shown in) :

Option- menu > about this Mac > (system Information) > network:

Aquantia AQC107-B0:

Name: ethernet

Type: Ethernet Controller

Bus: PCI

Slot: Slot-3

Vendor ID: 0x1d6a

Device ID: 0x87b1

Subsystem Vendor ID: 0x1d6a

Subsystem ID: 0x0001

Revision ID: 0x0002

Link Width: x4

BSD name: en2

Kext name: AppleEthernetAquantiaAqtion.kext

Location: /System/Library/Extensions/IONetworkingFamily.kext/Contents/PlugIns/AppleEthernetAquantiaAqtion.kext

Version: 1.0.64

Terminal command:

ifconfig en2 | grep media

with this as my output for 10 Gigabit Ethernet:

media: 10Gbase-T <full-duplex,flow-control>

For ‘regular’ Gigabit Ethernet, you should get this instead:

media: 1000baseT <full-duplex,flow-control>

There is Much more to the story, so please let readers know what you discover.

Launch the Terminal app. In the Terminal, run the named application and wait for it to finish with a summary:

This is on a Wi-Fi 802.11ax (5Ghz, 80 Mhz) connection to an M4 Mac Mini Pro running macOS 27. The Xfinity gateway is in an interior closet some distance away from my Mac, so no hardwired Ethernet to test.

If I run the Xfinity app on my iPhone 17 Pro Max also wirelessly connected to my Xfinity gateway, that app just reported the speed between the gateway and Xfinity head end at 1100 Mbps. Misleading because of all the signal blocking walls between my Mac and the gateway location.

Limitations for you might be the generation (< cat 5E) of Ethernet cable you are using; you installed any TP link drivers that may not be entirely compatible; if that Ethernet cable is damaged in some manner; your TP link adapter is not actually a 1000 Mbps rated device; or if your Ethernet connection is passing through a 100 Mbps rated router or switch which will automatically throttle your 1000 Mbps expectation.

Maybe you need another Belkin 1 Gbps adapter…

As MartinR I'd suspect the adapter.

FWIW I have MBP5 → Apple's Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter → Apple's Thunderbolt 2 Adapter to Ethernet Adapter → Ethernet and it works fine up to 500Mbps fibre downloads although I rarely use it because my router's Wi-Fi speed is just as good.

Thanks Grant Bennet-Alder, running these commands I've got the following output:

media: autoselect (100baseTX <full-duplex>)

I have an Anker A813 usb-c adapter directly connected to my M4 Pro Macbook Pro and it reports full speeds on our 300 MB/sec cable internet connection: