You could actually use SIMD instructions to detect sequences of bytes with the top bit cleared, thus allowing bulk copies of ASCII text without a per-byte loop.
Another optimization would be to take advantage of the fact that codepoint usage tends to cluster around the language of the text. So if you detect usage of 3-byte encodings, chances are you'll continue encountering only 3-byte encodings, with the odd ASCII or emoji codepoints. This opens up even more state machine possibilities.
You could actually use SIMD instructions to detect sequences of bytes with the top bit cleared, thus allowing bulk copies of ASCII text without a per-byte loop.
Another optimization would be to take advantage of the fact that codepoint usage tends to cluster around the language of the text. So if you detect usage of 3-byte encodings, chances are you'll continue encountering only 3-byte encodings, with the odd ASCII or emoji codepoints. This opens up even more state machine possibilities.