To get the best speeds you need the dongle outside facing the right way.
Ours is pretty optimal, spending a little time can produce some very good results:
Having a dongle indoors will reduce the speed. Same dongle, same location, but indoors on the window sill won't exceed 6Mbps, more usually 3 to 4Mbps.
Using a mobile phone instead which may well have a smaller less well optimised antenna and reduces it still further in the same way that our tablet PC gets a strong Wi-Fi signal in the house but mobiles do not. Have never seen over 3Mbps using a mobile phone (Samsung Galaxy S, Apple iPhone) here.
3 might actually prioritise the data differently (not sure on that one) reducing speed for tethered devices veruss dongles. Perhaps someone can confirm or deny that one.