Why glibc's fstat() is slow

On an adjacent topic, GNU coreutils also uses fstat in a way which seems unnecessary, e.g., compare the usage in its version of wc: https://github.com/coreutils/coreutils/blob/master/src/wc.c, heavily wrapping and processing fstat, with the busybox version (https://github.com/brgl/busybox/blob/master/libbb/wfopen_...), which reads from stdin by default or if if receives a "-" as the sole arg (see the definition of fopen_or_warn_stdin here: https://github.com/brgl/busybox/blob/master/libbb/wfopen_...).