Hi,
Currently DEM raster are read by the MicroStation raster engine prior to be converted into a surface.
Considering that DEM raster might not fit into memory and that Civil surface needs to be able to fit into memory it is often necessary to filter some elevation points coming from the DEM raster in order to create a surface that fits into memory.
After that the surface is treated as any other surface (i.e. : import operation). As far as I understand your performance issue seems related to the fact that you have surface created from DEM having a lot of points. One solution would be to filter more points during the import operation, at the cost of decrease precision.
HTH,
Mathieu