It does not actually create geometry on-the-fly. Instead, it uses a custom-build parts library, available open source here .
Max, don’t worry, it still progresses in the background. Note that only the sub-models themselves will be assembled. If you have to reimport the same model again, for example because you’ve added the missing parts or added new parts to the model, you can just run the importer again. It will ignore any part that was already imported and placed.In this case, you should remove the entire assembled model and run the import and assembly commands again. Otherwise, some parts will be scaled incorrectly compared to others. Reasonably complex models are reasonably fast to import, though running it on an already imported/assembled model will slow the importer as it has to check which parts already exist.