Linux is used on 54.9% of the world websites : almost every application running on a linux machine uses the glibc which provides the core libraries to access almost every feature of a linux system. The Mighty Glibc started back in 1988 and is a wonderful and glorious project.
As far as the string functions are concerned the sse / avx optimized versions of these functions (strlen, strcpy, strstr, strcmp and more) are up to 10 times faster than their corresponding standard c implementations (which for example you might find in the libmusl) when run on a sse/avx capable cpu.
We rely a lot on glibc string functions and that’s why we found that glibc 2.25 introduced some optimization on the AVX capable processors and this disabled sse* optimizations for methods that don’t have a avx2 optimized implementation (strstr, strcat, and I’m afraid parts of the math functions). For further details go here.
The bug affects ubuntu 18, debian 10, fedora 26 to 28.
A fix will come for sure, hopefully in glibc 2.29.