Why glibc's fstat() is slow

I despise the fact that POSIX insisted on standardizing the ugly TZ env variable as the standard interface for controlling timezones. It makes some amount of sense to have a process-wide value for "the local timezone." It makes considerably less sense to make that the *only* way of doing tz-aware calculations. If I want to know "what time is it in [some specific part of] Australia?", it does not necessarily follow that I want the whole process to think it is actually in Australia.