UB is very much a double edged sword for compilers. On the one hand, it makes lots of simple optimizations much easier, but on the other, it makes lots of inter-procedural optimizations much harder (since the compiler must be incredibly careful not to introduce UB that the user didn't introduce themself).

There is no compiler that actually uses all of the things that the standard allows them to do (especially wrt atomics), because if they did, everyone's code would break, and figuring out which code transforms were legal would be ridiculously difficult.

> at least compared to languages like Java and especially compared to languages like Ruby

I hope you didn't take from my previous comment that I think Java is a good language from this perspective. The fact that Java gets even gets half decent performance is a miracle given how bad the JVM model is. Ruby is a language I'm really interested to try out since IMO it was the language that first managed a modicum of optimization with python-like expressiveness.