Earlier quoted context omitted.
Ah, that's a shame to hear. FWIW, ChatGPT did also suggest that there was a lot of room for improvement in the MPS backend of PyTorch that would likely make it more efficient on Apple hardware in time.
You fundamentally misunderstand the bottleneck of large LLMs. It is not really possible to make gains that way. A 405B LLM has 405 billion parameters. If you run it at full "prescision", each parameter takes up 2 bytes, which means you need 810GB of memory. If it does not fit in RAM or GPU memory it will swap to disc and be unusably slow. You can run the model at reduced prescision to save memory, called quantisation…
The comment about the MPS PyTorch backend was related to performance, not whether the model would fit at all. I can't say whether it's accurate that the MPS backend has significant room for optimization, but it is still publicly listed as in beta.