lc() is rocket fast because this only involves fiddling with one bit within a byte.

That's only true for ASCII. For Unicode, things are quite different and will probably need some kind of table lookup. Just an example for the german language:

And unfortunately, that's not all. The LATIN CAPITAL LETTER SHARP S, as ẞ is called in Unicode, is very new. First proposals are about 140 years old, but it became part of the standard german orthography in 2017, after becoming part of Unicode in v5.1.0 (April 2008) and of ISO/IEC 10646 (June 2008).

Before that (i.e. up to 2017), converting LATIN SMALL LETTER SHARP S to upper case resulted in converting it to SS or SZ. And to make things worse, the conversion depends on context. SZ is always ok, but considered harder to read. In general, SS was (and still is) used, except where it may cause misunderstandings. A common example for that problem is "Maße" (dimensions) vs. "Masse" (mass), both can be upper-cased to "MASSE". The meaning can often be derived from the context, to avoid misunderstandings, "Maße" would be upper-cased to "MASZE". Reversing that (i.e. implementing lc()) opens another can of worms, as other words may contain "ss" or "sz" that must not be converted to "ß". The LATIN CAPITAL LETTER SHARP S solves that problem nicely, lc("ẞ") is always "ß", no funny context rules needed.

See also https://en.wikipedia.org/wiki/%C3%9F#Development_of_a_capital_form.

BTW, swiss standard german solved that problem differntly: ß was completely replaced by "ss" in the 20th century (mainly between the 1940s and 1970s). "Maße" vs. "Masse", both written as "Masse", has to be resolved by context.

Alexander