I got the impression that the A12's improvements over the A11 aren't easily measurable by spec sheet. Yes, it's just 15% faster; I'm sure that speed bump doesn't translate into any discernible improvement in, say, opening apps. GPU is 50% faster, but I don't know how best to view/demonstrate that speed increase. Maybe AR, or advanced games. Neither of which the average customer uses with any regularity (well, I don't).
However, some of the improvements include the 2nd gen neural engine stuff. Reportedly, the A11's neural engine was capable of handling 600 billion operations per second. The A12's can handle 5 trillion. Those numbers boggle the mind...I honestly don't know how these figures translates into real-world performance. However - I know Apple stated that this is what enables the Smart HDR features. Makes sense, because if you look at the Smart HDR process, it certainly seems like it's processor-intensive. So, this is one clearly visible improvement over prior HDR photo implementation.
There's also the fact that the A12 is a 7nm processor. Theoretically, this should allow the processor to operate more efficiently. Again, I'm not sure how you demonstrate that improvement, beyond (I suppose) longer battery life.
tl;dr - I think we have to wait to see how the A12's improvements shake out versus the A11. Maybe it will enable additional CPU-intensive features besides Smart HDR.