I was looking at a flame graph of an ingestion workload with @dliappis, and it looks like we parse the JSON representation of the index request twice when a document has both a pipeline and a final pipeline.

It's hard to know how much we would save on this benchmark since not all data streams in this benchmark had both a pipeline and a final pipeline, but the cost of parsing index request into maps of maps via IngestService represented 4.06% of overall CPU time according to the flame graph, so this might not be negligible.
server/src/main/java/org/elasticsearch/ingest/IngestDocument.java
if (index < 0 || index >= list.size()) {
                throw new IllegalArgumentException(
                    "[" + index + "] is out of bounds for array with length [" + list.size() + "] as part of path [" + path + "]"
                );
            }
            if (append) {
                Object object = list.get(index);
                Object newList = appendValues(object, value, allowDuplicates);
                if (newList != object) {
                    list.set(index, newList);
                }
                return;
            }
            list.set(index, value);
        } else {
            throw new IllegalArgumentException(
                "cannot set ["
                    + leafKey
                    + "] with parent object of type ["
                    + context.getClass().getName()
                    + "] as part of path ["
                    + path
                    + "]"
            );
        }
    }
    @SuppressWarnings("unchecked")
    private static Object appendValues(Object maybeList, Object value, boolean allowDuplicates) {
        List<Object> list;
        if (maybeList instanceof List) {