Glibc change exposing bugs

The glibc maintainers are much harsher than that. Any change which is implementation-defined in POSIX or ISO C's library functions is fair game for them to change, and they have very harsh words for people who complain (and explicitly don't care about breaking closed-source code that depends on such assumptions). Expecting them to insert slowing-down hacks for actively *undefined* stuff, given their somewhat cavalier attitude to implementation-defined stuff, is peculiar. (Their attitude to stuff that *is* defined is appropriately rigid: thou shalt not break it.)