On a tangent, I trust Apple's competence but I have seen no information on the technicalities of this feature. I would like to hear specifics so we can estimate how secure the design is based on facts.
https://www.apple.com/privacy/docs/iOS_Security_Guide_Sept_2...
The computational burden likely cannot be too high, as it is running on mobile hardware and people expect rapid access to their data after entering their passcode.
Going back to @theGimp's comment, it is probably safe to assume that nation-states that might want to decrypt this would have some form of access to that circuitry (either a specialized implementation or by using the apple chip) and could sidestep the speed issue. It seems like, to 0th order, it would either be slow for everyone or quick for everyone.