To not only use UTF-8 as the internal string encoding but practically mandate it, if you want to remain safe.
UTF-8 is a fine transport format, but for raw runtime performance it's obviously going to be an issue if you ever need to iterate over characters, do substring matches, things like that because you can't do constant time "next char" or indexing.
UTF-16 doesn't let you do that either in the presence of combining characters, but they're pretty rare and for many operations it doesn't really matter.
UTF-8 is a fine transport format, but for raw runtime performance it's obviously going to be an issue if you ever need to iterate over characters, do substring matches, things like that because you can't do constant time "next char" or indexing.
UTF-16 doesn't let you do that either in the presence of combining characters, but they're pretty rare and for many operations it doesn't really matter.