Six cores are quite a few, and Swift doesn't have good concurrency support yet (arguably one of the big pieces still missing). And even if it did, so far efforts to make multithreading more pervasive haven't had a great yield.
This year I expect them to focus on energy saving more than speed. iOS 12 is all about performance, the system will look faster even without a big speed increase on the CPU side.
And GPU is pretty important for ML, games and AR. I think they'll try to increase GPU performance more than CPU in the future.