Wi-Fi painfully slow when in Clamshell mode

I'm considering buying a Twelve South BookArc Desktop Stand to save space when at my desk (I use an external display, keyboard, and mouse at my desk anyway). Available from Apple Store, the BookArc holds a MacBook in clamshell mode (vertically on end).

However, in learning how clamshell mode works, I discovered my network speed drops significantly when in clamshell mode. For example, I just ran tests with the lid open and in clamshell mode using Speedtest.net. With the lid open, my download speed was *18.49 Mb/s*. In clamshell mode, my download speed was *0.49 Mb/s*.

I then tried running the tests multiple times with lid open and in clamshell mode and consistently saw similar results. I also tried holding the MacBook vertically in clamshell mode so the display is in the same position it would be when open--just to be sure it wasn't an issue with the angle of the internal antenna. Similar results. Strange thing is, +if I start a Speedtest.net test in clamshell mode then open the lid during the test, the speed quickly jumps up again+.

I could live with a small decrease in network speed, but this is ridiculous. Wondering if its a problem specific to my MacBook, or a known issue with clamshell mode. Can anyone else reproduce?