In such cases I would think the optimization is basically for ASCII, and then extend that to Latin1 because it happens to be the first 256 characters of Unicode

In such cases I would think the optimization is basically for ASCII, and then extend that to Latin1 because it happens to be the first 256 characters of Unicode, which means processing is trivial and you don't waste the other 128 byte values that way. But I figure that 99.9% of those strings would be ASCII-only.