This is the best writeup I found: http://blog.trailofbits.com/2016/02/17/apple-can-comply-with...

So, you'd need an update to iOS/the phone firmware, and for newer devices you'd also need an update to the secure enclave firmware. You can't do anything about the 80ms delay, because that's baked into the hashing function (and changing the hashing function would generate invalid results). The FBI is also asking for the ability to enter passcodes electronically rather than via the touchscreen, which would be new code.

If iOS and the SE firmware are really nicely factored to disable security, and it's not hard to add the new functionality, then this might not be too much work. However I doubt that that is going to be the case. The whole point of the security system is to make it difficult to crack, so there might be other countermeasures involved, tricky dependencies, and low-level hardware hackery. If it were simple to do, why wouldn't it have already been done by others reverse-engineering the compiled code? There is certainly financial motivation to do so.

I'm willing to bet iOS has a huge build infrastructure, many different components, and about a snowball's chance in hell of having a single nice clean Makefile for you to type one command to get a build without access to that infrastructure.

People are making it out like the FBI is asking Apple to rewrite a big part of iOS. That's not the part of the request that's the problem.