Thanks a lot Karl for suggested performance optimization, much appriciated. In fact I knew the implementation of the POW is not optimized, I haven't got around to it yet.
But that is irrelevant to this thread I believe? It is independent? Since the fact remains, that the same code using either CryptoKit or CryptoSwift results in a big difference in performance...
Anyway, Apple followed up a couple of days ago, so awaiting response from them.
Thx guys!