Performance comparison iOS 12 / iOS 11?
To my knowledge, no such measure exists.

It is virtually impossible to say anything about the performance of an operating system that is fully independent on the processor and network. It really doesn't make much sense.

You can create benchmarks that measure the performance of "real life things", which includes stuff the operating system does - but you wouldn't be able to compare the OS performance across different processors and network devices.

It would make more sense to create a benchmark that tries to focus on operating system operations - trying to spend as much time in the OS as possible and as little in user code. Then you could compare that benchmark value to the same benchmark value on the same exact device (i.e. same processor, etc.) but with a different OS version.

This would tell you a little bit about the OS performance, but it would be very hard to create something that you gives you 1 abstract number that represents the OS performance in a meaningful way. It would depends very much on heuristics and how you want to weigh the performance of each of the many thousands things the particular OS does for you.

For example if iOS 11 is 20% faster at creating threads compared to iOS 12, but iOS 12 is 20% faster at compressing video - which one is the fastest?

If you never compress video, you might say iOS 11 is the fastest, but others might have a different opinion.