I do write and profile software of that kind and this experience is why I know this isn't a myth. Any mature program has a whole lot of code that actually isn't performance critical at all. For example, 3d software needs a really huge amount of GUI and other support code that isn't performance critical at all. The performance hotspots are really just individual functions doing the core of the processing work for any of the features it offers. The initiation/scaffolding code around that just doesn't matter. The same translates to all other software that that I have worked on.