Same results on a BlackBerry...
I took interet in this thread because I have been suspecting for some time now that I get faster speeds on my phone than I do when I tether. Eligible for my NE2 discount, I am still debating on which Android to get, spending the big $$ on the Thunderbolt or going potentially free with Droid Incredible or Droid X. Anyhow, that's a discussion for another thread, but I did want to share my results for use on a BlackBerry, to offer an opinion that this may not be just a Droid issue.
I ran mobilespeedtest.com's speedtest on my phone and got 484 kbps. Running on the same test via Tether, I got 117 kbs. Same 3G connection but the tethering is much slower. The reason I have been suspecting a slower speed is that I have no problem viewing video on my phone, but I can't seem to view video online via tethering. I figure that video is more data intensive, thus if I got a slower speed online than I do on my phone, that may explain why I get multiple buffering data alerts when I try to view online video via tethering.
@rosser, thank's for the tip on mobilespeedtest.com. It seems much more reliable than speedtest.net. I am going to try it on the Thunderbolt when I go back to the Verizon store. Maybe I won't get speeds like 12 Mbs down & 30 mbs up. Maybe, just maybe, I'll get the "true" speeds of the phone, like 3 Mbs down and 0.5 mbs up!