Remember mercurial? Me neither. But what I remeber is this article I've read about all the hacks they had to do to achieve reasonable startup time for CLI in python. And the no #1 cause was loading the whole world you don't ever need. As I recall they somehow monkeypatched the interpreter to ignore imports and just remember their existence until they were actually needed, at which point the import happened. So all the dead paths were just skipped.