By driving at 20 mph when the posted speed limit is 45 (and there is no reason to do so---rain, snow, sleet, etc), you ARE impeding traffic. It is dangerous. Let me give you an example:
You are driving 20 mph. You go around a curve or down a hill. Another car, in the same lane, doesn't see you (b/c of the curve or hill) and they slam in the back of you. They don't see you b/c they are approaching the top of the hill, or the curve. Since this other car is driving 25 mph faster than you, they will hit you fairly quickly. They can't stop b/c of the difference in speed. They are not being careless or unsafe. YOU are. YOU caused a wreck that could have easily been prevented.
As far as the harrassment, put yourself in their position. As a police officer, you do not know who is safe and who isn't. They do not know what your intentions are. We have seen your attitude. You probably displayed that to the officer, thus confirming his/her opinion.
But, you won't learn. Your attitude says it all.