I suggest to remove the (now <= 0) check.

        _PyTime_t now = _PyTime_GetPerfCounter();
        if (now <= 0) {
            Py_FatalError("_PyTime_GetPerfCounter() == 0");
        }

Please don't add such Py_FatalError() in the middle of a function. If this case happens, it means that Python immediately exits and creates a debug report (ex: coredump on Unix, popup on Windows asking to open a debugger), and worse: it exits the process immediately.

I'm not aware of anyone reporting that _PyTime_GetPerfCounter() returns 0. Do you fear that the clock is broken and *always* report 0? If yes, please add at check when Python starts. But why do you think that it could happen?