If you look at the Linus's trace, it's all heap and mutex operations. That's just sloppy internal design full of concurrency bottlenecks and lots of in-memory cloning. You certainly don't mean to imply that both are the only way to implement a "security policy driven message bus" software, do you?
Far from it. One of the core designs of kbus (which dbus cannot do because it's not in the kernel) is that you can seal the payload buffer from the sender so the receiver and use it safely concurrently without having to clone anything.
There is obviously a lot of general inefficiency in the userland libraries but not in the design of it.
That was created for KDBUS but has more use than than being used for KDBUS exclusively. It still was written for KDBUS.