It seems a Thunderbolt/USB4 external NVME enclosure can do about 2500-3000 MB/s which is about half of internal SSD. So not at all bad. It’ll just add an additional few tens of seconds while loading the model. Totally manageable.
Edit: in fact this is the proper route anyway since it allows you to work with huge model and intermediate FP16/FP32 files while quantizing. Internal storage, regardless of how much, will run out quickly.
This only applies to Macs and Mac-a-likes. Actual desktop PCs have many SATA ports and can store reasonable amounts of data without the crutch of external high latency storage making things iffy. I say this as someone with TBs of llama models on disk and I do quantization myself (sometimes).
BTW my computer cost <$900 w/17TB of storage currently and can run up to 34B 5bit llm. I could spend $250 more to upgrade to 128GB of DDR4 2666 ram and run the 65B/70B but 180B is out of the range. You do have to spend big money for that.