A sample use:
[...]
'include_once' allows me to lazy load -- and thus optimize my
application. Sure, it may be a slight performance hit -- but not nearly
that of loading the files in question.
I do not question usefulness of include_once(). In fact, currently I'm developing application that might use it exactly the way you described. There are many API functions in several files, and there are cases when I do not need all of them. That is exactly why I am concerned with it's performance, and that is why I asked about it here.