Swift 5ではStringをUTF-16ではなくUTF-8で保持する様になるらしい。 https://twitter.com/jckarter/status/1059586744083988482

Thanks to @Ilseman's tireless work, Swift 5's String will use UTF-8 as its native representation, with great performance improvements! https://t.co/CYeCToAKUf