A researcher named Eddy Zervigon is quoted saying cryptocurrencies are "the canary in the coal mine" for quantum computing, since a working attack would show up on public chains before it showed up at a bank. That part follows from the design; the ledger is public, so a break is public too. His firm's own point is the sharper one: a bank needs a board resolution to change its cryptography. A network needs near-unanimous agreement among strangers. Consensus was always the slow part. It was supposed to be.