Why does at&t edge have such a high latency (average 800ms)?

I don't mind the bandwidth I average around 0.15mbps down. I use my iphone 4 to tether (currently 3G is still not offered in my area) But the lag kills me on anything I don't see how it's physically possible for it to lag so bad back in the day (5 years ago or so) it was pinging at 300ms but I don't quite get it I'm in the middle of nowhere. Can anyone shed light if there is a fix for this? Or why it would get that bad over the years?