Sounds like an incredibly complex, slow performing townhouse. You may want to consider separating the ground floor from the first and saving each as different models. Essentially becoming two individual models and two separate walkthroughs, cutting the project size in half, or near there hopefully. To be honest, I've never heard of or seen a model this heavy, at least when compared to the volumetric size of an average double story townhouse. The largest model I've ever worked on was around 1.4gb consolidated, was never edited as a singular model and had 85 storeys at 200,000 sqfpf. Do you mean 90 mb?