It doesn't have to be implemented as a message bus. It should just be a message bus semantically. Under the hood we could implement all sorts of tricks to make it faster, as long as the applications don't notice the difference.
I would expect that to work to a point, but coherence and interlocking are eventually going to rear their ugly heads.
I've created and lived with multiple inter-process high-performance multimedia/data systems (e.g. video special effects and editing, real-time audio processing, bioinformatics), and I've yet to encounter a message passing semantic that could match the performance of manually managed systems for the broad range of non-pathological use-cases, not to speak of the broader range of theoretically possible use-cases.
If something's out there, I'd love to see it. So far as I know, nobody has cracked that nut yet.
