Glenn Fleishman, writing for MacWorld:

Starting with the A7 processor, Apple’s use of a Secure Enclave chip pays dividends in resisting brute-force and other methods of cracking an iOS device’s passcode. (The A7 first appeared in the iPhone 5s, iPad mini 2, and iPad Air. All subsequent iOS devices and processors include this support.)

With Secure Enclave, even a relatively weak passcode or passphrase is combined with enough information stored uniquely in the phone that can’t be retrieved to require an extremely long period of time to determine the correct password. As [Cryptographic expert and university professor Matthew] Green notes in his post, Secure Enclave means that every password-cracking attempt has to happen on the iOS device; the part that needs to be cracked can’t be exported and iterated against on another system, like a set of high-performance graphics cards—or an NSA supercomputer.

In addition to more technical details, Glenn digs into the political side as well. The whole piece is interesting, well written.