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
list = (List<Object>) maybeList;
        } else {
            
            list = new ArrayList<>();
            list.add(maybeList);
        }
        if (allowDuplicates) {
            appendValues(list, value);
            return list;
        } else {
            
            return appendValuesWithoutDuplicates(list, value) ? list : maybeList;
        }
    }
    private static void appendValues(List<Object> list, Object value) {
        if (value instanceof List<?> l) {
            list.addAll(l);
        } else {
            list.add(value);
        }
    }
    private static boolean appendValuesWithoutDuplicates(List<Object> list, Object value) {
        boolean valuesWereAppended = false;
        if (value instanceof List<?> valueList) {
            for (Object val : valueList) {
                if (list.contains(val) == false) {
                    list.add(val);
                    valuesWereAppended = true;
                }