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.