Naive REST (HTTP, JSON) isn't exactly known for its performance
Naïve XML isn't known for its performance either. JSON is usually faster to parse than XML.
While D-COM uses a binary protocol, it uses XML for its "Introspection Data Format". Objects instances may implement Introspect which returns an XML description of the object, including its interfaces (with signals and methods), objects below it in the object path tree, and its properties. Objects may be introspected at runtime, returning an XML string that describes the object. The same XML format may be used in other contexts as well, for example as an "IDL" for generating static language bindings.
More info: http://dbus.freedesktop.org/doc/dbus-specification.html
In Plan 9, each process has its own filesystem tree, and other programs can expose themselves to this process as file servers, meaning that data internal to the programs can be accessed via the same read, write, delete, etc. calls as files. For example, when running under rio, the Plan 9 window system, the current window contents are available at /dev/window and you can write draw calls to /dev/draw to do graphics.
REST is pretty much applying Plan 9 principles to the Web and http.