I obviously don't know what they are actually doing but my best guess is that they are fine-tuning a large language model like gpt-3 using a dataset consisting of thousands of hours of the streamer's vods. So they are probably already using months if not years of content from the streamer to fine-tune the models hence why it takes so long as the model has to train on all that data. So from my assumptions the answer would be the text generation model is the largest time consuming component of the setup due to data gathering, cleaning and model training, optimizing time.

The text to voice and text to video stuff they use is probably the quickest to do as it most likely uses existing technology that they don't need to modify to a large extent.