Do YubiKeys decrypt GPG or SSH/SFTP data directly within themselves? If so, would that cause a bottleneck? Do YubiKeys have accelerated AES hardware that does most of the heavy lifting of decryption?
Also, YubiKeys aren't needed when encrypting to another person's key, correct?
The YubiKey only does public key cryptography, not symmetric cryptography.
In each of those cases, the YubiKey is used to encrypt (or decrypt) a symmetric key, which is then used by a symmetric cipher such as AES. The YubiKey is not involved at all, after this key is made available. Symmetric cryptography is performed by the device (computer, phone, etc).
