The "deal" is most likely the file construction. Often a "line" is not simply a line when opening cad files. There can be stacks of many paths overlapping each other. The best method is to remove all superfluos items from the CAD drawing before attempting output.
Beyond that it's difficult to say without seeing the file.
Agree with what Scott said. It may just look like single lines but due to how CAD programs construct the visual appearance based on drawing primitives rather than "paths" as it is understood in AI, there could be an awful lot of them in the same place even in seemingly simple drawings. I'm sure you'd see the same performance issue if you attempted to manualyl flattening the file in AI... You will have to go in there and clean it up or even better, clean it up in the CAD program and export it differently from there...