It's not coded in the client, when you create a passphrase it appends a random salt at the end for it so that it will take on average 0.1 seconds to try and decrypt.

Example: your password is password.

Your computer can do 20,000 decryptions per second.

So your real password would be between password000 to password999. (So on average 0.1 seconds, because on average you're going to decrypt it halfway through).

When you decrypt, bitcoind tries all the keyspace which on average will take 0.1 seconds.