It may well be slower, but how is it not scalable? You can always add more servers, or more Yubikey devices. In any case, I would expect any slowdown to be sim

It may well be slower, but how is it not scalable? You can always add more servers, or more Yubikey devices. In any case, I would expect any slowdown to be simply because the Yubikey is slower at doing cryptographic operations, not because of any overhead associated with USB. On my system, generating a 4096-bit RSA signature with OpenSSH takes about 4ms, whereas a bit of googling suggests a no-op USB round-trip is on the order of 200us.