Very low. With good SNR you can get 48.0 kb/s. It basically just passed small text payloads across an 'air gap', that were bigger than you'd want to be typing

Very low. With good SNR you can get 48.0 kb/s. It basically just passed small text payloads across an 'air gap', that were bigger than you'd want to be typing by hand. They were encrypted in transit but inspectable on both sides. Shielded cable was long enough to reach a separate isolated room, and easily unplugged by an operator when not in use. USB wasn't considered secure enough, other forms of sneakernet were avoided for *reasons*. Alternative options could have been used but this was simple, auditable and (comparatively) easy. Stakeholders fully aware of how it works and the weirdness factor. Not many users, and no scale involved. It just met a very niche use case nicely :-). Hope that didn't dispel the magic for you.