I had a quick look at the original dxf and it is made from polylines. Most post processors cant understand them and break them down into loads of little lines. That way you get the kind of effect your are getting with it. Curves end up being made from straight lines. This also causes machines to stutter when trying to process the code also prevents them from reaching top cutting speed due to the amount of code they are trying to load and run.
Take your original drawing and then draw over it in cad using lines and arcs only. Then save it as a dxf and post process it again and try the results. Hopefully you will get a nice smooth contour.