I connected my Wii U via wifi and got 5 mbs for download speed at http://www.bandwidthplace.com/.
Thinking this was slow (my iPad tested 18 mbs on the same test) bought the rocketfish wired adapter for the Wii and ran a cable from my router. I ran the same test and only got 2 mbs.
Any idea why wired would be slower than wifi? It seems really backwards to me. I switched back and forth between wired and wifi several times and the wifi was always faster.