Run the "Visibility of objects on floors" command in the settings menu. In the dialog, there are options to show the upper floor / lower floor in Static and Architectural mode. While these options are active, you can see the traces of other floors as a dashed line on the active floor plan. If you turn off these options and export, the lines you mentioned will not appear in the DWG file. There is no extra slowness in DWG. "Invalid line type" error is given for undefined line types imported from another dwg file. This export does not create a problem.