From my own experience i would say that tethering does not slow down the connection by a noticeable amount.
I have the iPad 3G and when I tether it via wifi to a laptop, the laptop actually loads websites and downloads faster, then when doing the same task on the iPad. So obviously the iPad is the limiting factor in determining the overall speed and not the tethering connection. The iPad 2 will be faster, so this might not be completely accurate, but I wouldn't expect much of a speed difference.