UB is an implementation issue, not a language issue. UB is a fact of life that cannot be removed by overspecifying a language -- if your language spec has too much implementation-specific stuff in it, then compiler writers will simply ignore it. Many such examples. The best you can do is to document the implementation-specific stuff extensively.

Also, implementation-specific details aren't always about maximizing performance. Compilers implement languages slightly differently because of architecture and OS differences and because it's easier that way for the people writing compilers.