I use a thunderbolt to ethernet adaptor at work on a Macbook Pro and have seen ~ hundreds of Mbit/sec speeds routinely on downloads from the internet through this adaptor.
Are you comparing the same things here, e.g. max download speed over the internet, say through Speedtest.net or some similar site? Or are you comparing internet via the ethernet/thunderbolt adaptor with a transfer wireless just over your local network? What is the limit on your internet connection? 20 Mbps sounds typical of max internet speeds for some providers. On the other hand, if both your tests involve just local network transfers, then you need to look at how your router is configured for ethernet versus wireless so see if there is some constraint being applied that you don't want.
I suggest two tests, each done under similar conditions (e.g. no other network traffic) with (a) ethernet wired with thunderbolt adaptor and (b) wireless:
(1) speedtest.net -- to test internet speeds
(2) a local copy thruogh your local network of a large file/folder at least 1 Gigabyte in size to test local network speed.
Steve, thanks for the quick reply. Sorry for the vagueness.
Both tests were performed using speedtest.net on my 2012 MBPr. One with N wireless, and one through a wired connection using the Thunderbolt to Ethernet adapter.
As a reference, I also compared my 2008 MBP that has an Ethernet port.
The 2008 MBP is much faster in wired download speed compared to my new 2012 MBPr, but the MBPr does have much faster wireless speeds. I was expecting similar speeds using a wired connection.
I wonder whether my adapter is bad, or my Thunderbolt io is bad.
Looks like a reboot solved the artificial ceiling I was seeing in my Thunderbold to Ethernet adapter.
Now, both the 20012 and 2008 MBPs are acting similar with a wired ethernet connection.
The 2012 MBPr has superior wireless speeds, very similar to a wired connection.
Using Comcast Telecomuter business class, I see an average of 70Mbps downstream,
and 30Mpbs upstream using Speedtest.net.
Thanks for your hlep.
One more thing -- the thunderbolt/ethernet adaptor supports Gigabit ethernet (as borne out in the examples I gave in my earlier post). Some routers are limited to 100 Mb/s wired. Sometimes a router is the source of the bottleneck, e.g. it might be faster via wireless than wired due to its own internal limitations. It sounds like your reboot solved your issue, but I'm mentioning this in case others are interested.
With ethernet/thunderbolt adapters, there are 10/100 models (like the one Apple is selling), or 10/100/1000 models (which are, of course, much faster).
I see your problem was software/connection related.
Just in case, here is a 10/100/1000 model at $ 25.00 plus tax:
For anyone else reading this question, I had the same problem until I easily fixed it (with Apple help).
If your computer acknowledges your Thunderbolt/Ethernet connection, go to Advanced.. > Hardware.
Then change the Configuration to Manually.
My Duplex was on "full-duplex, energy-efficient-ethernet". When I changed it to plain "full-duplex" my Mbps went from 0.5 to 60.