That's what I meant by "must be O(1)". I.e. constant time. String's Index type puts the non-constant cost of identifying a grapheme's location into the index creation functions (`index(_:offsetBy:)`, etc). Once you have an Index, then you can use it to subscript the string in constant time.

Like I said, you can easily extend String to look up graphemes by integer index, but you should define it as a function, not a subscript, to honor the convention of using subscripts only for constant-time access.

It's also just not a normal use case. In ten years of exclusive Swift usage, I've never had to get a string's nth grapheme, except for toy problems like Advent of Code.