Namely, that when a user voted in a poll, the new vote AND ALL previous votes were being reverse replicated. This caused a MASSIVE workload on the Author because each node in the /var/replication/outbox did not contain 1 corresponding vote; it actually contained ALL of the votes including the new additional one. This explains why the Author would take 20 minutes to process just 10 nodes in the outbox.
The root cause was in the structure we were using (has been abbreviated):-
Each vote is added under the /answers node as type "nt:unstructured" with the various properties. But, on each submission of a vote, the custom code (a custom SlingPostServlet) was setting the 3 magic properties (cq:distribute, jcr:lastModified & jcr:lastModifiedBy) on "/poll1/jcr:content". This causes the page "poll1" to be marked for reverse replication - and with it, all it's sub-nodes (aggregated).
The solution was to change the nodes that get created to individual pages themselves as follows :-
And, then to ensure that the 3 magic properties are created on the jcr:content node of each vote node. Note: DO NOT have a jcr:content node anywhere in the intermediate hierarchy because this interferes with the firing of the outbox manager (I think it sees a jcr:content node and assumes that there must be a page there but, because there isn't a page there, then it aborts - and nothing appears in the outbox. I suffered with this problem when I kept "poll1/jcr:content" in the path (i.e. /content/usergenerated/somepoll/poll1/jcr:content/question/answers/1/12423434/jcr:content).
NB, due to our environment, we needed to use a custom SlingPostServlet and start the reverse replication in our project. However, the above structure should work with the normal OOTB page manager activated reverse replication.
The HTML form to post these votes would be something like this :-
<form action="/content/usergenerated/somepoll/poll1/question/answers/1/123456789" method="post" enctype="multipart/form-data">
<input type="hidden" name="./jcr:primaryType" value="cq:Page" />
<input type="hidden" name="././jcr:content/jcr:primaryType" value="cq:PageContent" />
<input type="hidden" name="././jcr:content/answer" value="my_chosen_answer" />
<input type="hidden" name=":redirect" value='/content/website/thankyou.html' />