Threading is indeed a performance hack - the whole idea of multiple processes sharing an address space is pretty gross and it's very unsurprising that it leads to many bugs - but there's also a reason why it was universally adopted despite the problems. I do think there's a fair argument that libc's design was not a mistake; the thread-unsafe functions worked very well in the environment they were designed for and it wasn't until decades later in a very different environment that they became a problem. Nonetheless, they are a problem now.
