How fast can you fix a checkout that is completely down?

Fast. On emergency tickets we open triage and aim for a first diagnosis during my working hours (10 AM to 7 PM IST, Monday to Saturday). Many checkout-down issues, a stale nonce, a cached checkout page, a single fatal in a gateway plugin, are resolved the same day once we can reproduce them. We always reproduce on a clone first so the live fix is verified before it goes out.