That's what I meant by "high latency". Workflow would be something like go to sign up to a site, site issues a challenge which is stored in your browser, then sometime in the next week/month/year you stop by the vending machine which generates a proof for the challenge, then you can finish the signup flow for the site in the next week/month/year.

Of course, this would require people to exercise some restraint with regards to their timing.

But the real problem is that nobody actually wants these types of systems, so there is no organic demand. The motivation only comes as directives from governments, so it's not about the technically best system but rather whatever corporate lobbyists can manage to get mandated.