[JonChesterfield]

Optimization means "make it faster without changing behaviour in ways I don't like". Clang can't generally default that one to on because it doesn't know whether you're going to splice in more code it can't see at runtime.

Lots of code gets slower if it might need to be called from something not currently in the compiler's scope. That's essentially what ABI overhead is. If there isn't already, there should be a compiler flag that says "this is the whole program, have at it" which implies the vtables option.