I have just ordered one of the new Nov 2018 MacBook Airs that only come with Thunderbolt 3 (USB-C) ports. Just looking for a simple clean solution for a high performance adapter using the USB-C physical connector.

Notes:

1. I am aware of the many USB-C to gigabit ethernet adapters that exist — but I am looking for an adapter that takes advantage of Thunderbolt 3, the lower latency, and lower overheads. For example, on my old MacBook Air (2014), I use a Thunderbolt to Ethernet adapter —> NOT a USB to Ethernet adapter.

2. I am aware I can buy a Thunderbolt 3 to Thunderbolt adapter, and then plug in my Thunderbolt to Ethernet adapter … but I don't want a double dongle. Who does?

Does there exist a Thunderbolt 3 to Gigabit ethernet adapter?

(n.b. the rest of this should be made into an answer and edited out including this editor's note…)

Note on the USB-C to Ethernet that Apple sells (Belkin)

If you read the reviews on the USB-C to Ethernet adapter that Apple officially sells (made by Belkin), there are a number of reviews that suggest that speed is throttled by using the USB-C adapter (compared to Thunderbolt).

A review by Elias A states:

"Compared the old macbook dongel that has thunderbolt with this one. Got close to 1000Mbits. With this Belkin USB-C dongel I get around 200-300 Mbits"

A review by William S states:

Not really gigabit
This adapter is only recognized as a 100BaseTX, even if you try to set it manually. Returned it and bought a Thunderbolt 3 (USB-C) to Thunderbolt 2 adapter since I already had an old Thunderbolt to Ethernet adapter from Apple. This combo may not be elegant, but it gives me full gigabit speed

It is these reviews that prompted my question. There are other reviews that note that Thunderbolt also involves lower system resources than USB-C.

Update after buying Belkin USB-C to gigabit Ethernet adaptor

Based on one of the answers below, I unfortunately purchased the Belkin USB-C to gigabit ethernet adaptor … and report below some comparison timings which reflect very poorly on the product.

In the following tests, I compare the speed to transfer, on the same Cat6 ethernet network, over the same cables, through the same routers:

• 2014 Macbook Air: using Apple Thunderbolt to Ethernet adaptor
• 2018 Macbook Air Retina: using Belkin USB-C (plugged into Thunderbolt 3 port) to Ethernet adaptor

Test 1: 3.8GB file

• 2014 Mac – Apple Thunderbolt adaptor: 35 seconds
• 2018 Mac – Belkin USB-C: 110 seconds. (Repeat tests returned 35 secs)

Test 2: 9.6 GB folder (with many files)

• 2014 Mac – Apple Thunderbolt adaptor: 5 min 50 seconds
• 2018 Mac – Belkin USB-C: 6 min 45 seconds

Test 3: 4 GB movie

• 2014 Mac – Apple Thunderbolt adaptor: 63 seconds
• 2018 Mac – Belkin USB-C: 87 seconds

The Belkin adaptor also got very very hot, and smelled burnt. I cannot explain the wildly variable results from the Belkin product either. An adaptor that returns worse results on a much faster 2018 Macbook Air (using USB-C) than Apple's own Thunderbolt adaptor on the 2014 Macbook Air frankly beggars belief. Whether the fault is the Belkin unit (of which there are many complaints on the Apple store site) or some inferior aspect of using USB-C compared to Thunderbolt … I cannot say.

I will be returning the Belkin unit to the Apple store for a full refund: the product or technology is plainly inferior to what was available 4 years ago.