Also the narrower niche of C++ means that it often makes sense to rewrite a small part of a Java app in C++, but it rarely makes sense to write a small part of a C++ app in Java. This domain difference can also explain the relative frequency of public "made it faster with C++" posts, since those small self contained uses make good posts that aren't entangled with the bigger application.

A couple of times as far as language implementations go (eg JRuby vs CRuby, even before the whole Graal/Truffle thing). However, that's a niche thing, and any JVM implementations have yet to replace the standard runtimes.

As a rule of thumb, I agree that C++ should generally be considered the 'faster' language.