I should probably have been more explicit about why I am asking the question, it isn't about overheads within a node, it is about sequencing of message handling as the flow progresses. In this flow
When A passes on a (reference to a) message it will be processed by node B and then by Node A, or perhaps the other way around, it depends on what order the nodes are wired up. Therefore it is not safe to make assumptions about which one will be run first.
In this flow, however
It is guaranteed that both B and C will be run before D gets hold of the message passed on by C.
Now if C is replaced by link nodes so we have
Then if the link nodes are just a drawing trick and the result is the same as drawing a wire directly between A and D then once again one cannot rely on the order of execution of B and D, but if they are equivalent to nodes that do effectively nothing except pass the message on then one can safely assume that B will be run before D.