As I understand it UB was not really intended to be for optimisation. It was so that C could compile on wildly different architectures that existed at the time.

Today we don't have nearly the variety of architectures, so they in theory C doesn't need nearly as much UB (like more modern languages).

Although there is one modern case where C's "anything goes" attitude has actually helped: CHERI works pretty well with C/C++ even though pointers are double the size they normally are, because doing so many things with pointers is UB (I assume because of segmented memory). CHERI is a slightly awkward target for Rust because Rust makes more assumptions about pointers - specifically that pointers and addresses are the same size.