It's no longer quadratic in so many cases, but it's still true that UTF-8 string operations require, in the best case, several CPU cycles per character consumed, even when the input is an ASCII subset. LC_ALL=C pretty much guarantees one or fewer CPU cycles per input character. Basics like strlen and strchr and strstr are significantly faster in "C" locale.
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Gnu grep is or was very slow with the UTF-8 locale. Not sure about other commands, perhaps anything that processes text, awk and sed maybe?
That was mostly fixed. http://savannah.gnu.org/bugs/?14472
It's no longer quadratic in so many cases, but it's still true that UTF-8 string operations require, in the best case, several CPU cycles per character consumed, even when the input is an ASCII subset. LC_ALL=C pretty much guarantees one or fewer CPU cycles per input character. Basics like strlen and strchr and strstr are significantly faster in "C" locale.