Performance vs security
For further reference, see
http://en.wikipedia.org/wiki/Advanced_Encryption_Standard#Security
http://en.wikipedia.org/wiki/Biclique_attack
http://en.wikipedia.org/wiki/Secure_Hash_Algorithm

For the time being, both AES-256 as well as SHA-512 remain relatively secure and overkilling the key derivation and thus slowing the performance down is quite frankly, well... overkill.