Sometimes cad models contain too much information
and need optimisation. Usually there´s too much
polygons which are not useful for rendering.
Less polygons are useful for rendering because
there´s so much detail in file model that
light rays and shadow takes longer time
to calculate with saw toothed edges.
File surface can be also textured
with just a simple gradient shader with evenly
distributed color knots on diffuse channel.
This method will save some rendering time
and you will be happy after fixing your
models. Optimisation will also make your
model lighter to handel and
have less memory cost in project.
You can use polygon reduction tool or
just Cinema 4D´s polygon modeling tools
to delete, optimize and melt any
polygons. If you need to model new geometry then
just use polygon, bridge and close polygon
demo: File model from 10000 to 200 and added texture.