The DIALux calculation kernel is able to handle about 20.000 surfaces within a project. The handling might become quite slow and crossing the limit the calculation kernel could break down entirely. When reaching the critical limit of the maximum manageable number of surfaces the calculation needs quite a long time. For the calculation of the amount of indirect light the radiosity method is used. This means that every surface interferes with every other surface. Just imagine how many calculation steps are required. So please try to reduce the number of surfaces, where possible. The key is to find a good compromise between a realistic visualization and moderate quantity of surfaces. Maybe you can make use of suitable textures, as in this way you can improve the visual impression of a less detailed object.