Try to keep the size of imported files to a minimum. Importing very large CAD files can take a long time because each CAD element must be analyzed and converted into a SketchUp entity. Also, once imported, a complex CAD file can slow SketchUp's performance because lines and faces in SketchUp have a lot more 'intelligence' than their CAD equivalents.
Strategies for Minimizing CAD File Size
SketchUp models can be designed to be as accurate as models in CAD. However, SketchUp is not designed for the same type of line-intensive drawings done in CAD software. Therefore, it is a good idea to clean up and import only the CAD content that is absolutely necessary into SketchUp.
Another strategy is to keep different levels of detail in separate CAD files. For example, one imported CAD file can contain site plan information, another can have a floor plan, and a final file can have a specific detail.