Reading a file is actually slow.

getCurrentThreadUserTime() uses many syscalls because it reads from /proc.
clock_gettime(CLOCK_THREAD_CPUTIME_ID) has only one syscall and a direct function call chain.

The optimisation can be done, but:

1. The kernel policy is clear: don't break userspace
2. It's undocumented anywhere!
3. Author's take: if glibc depends on it, it's not going away.

This is why I like browsing commits of large open source projects. A 40-line deletion eliminated a 400x performance gap. The fix required no new kernel features, just knowledge of a stable-but-obscure Linux ABI detail.

The lessons:

• read the kernel source. POSIX tells what's portable; the kernel source code tells what's possible.
• check the old assumptions: revisiting them occasionally pays off.