With bluetooth, you're doing pretty well to be getting 0.5mbit, this is bluetooth's fault, not the n900's. With the cable you should have no problems and i'd be more inclined to suggest the slowdown is your provider more than the phone, you may wish to make sure there're no bottlenecks in the actual tethering process though, which you can do by running a speedtest on any number of connection testing websites on your n900 and then on your mac while tethered via the cable, and see if the speeds are the same.