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
* @param primary the primary map
     * @param overrides the overrides map
     */
    private record DelegatingMapView(Map<String, Object> primary, Map<String, Object> overrides) implements Map<String, Object> {
        @Override
        public boolean containsKey(Object key) {
            
            
            return primary.containsKey(key) || overrides.containsKey(key);
        }
        @Override
        public Object get(Object key) {
            
            
            Object result = overrides.get(key);
            return result != null ? result : primary.get(key);
        }
        @Override
        public int size() {
            throw new UnsupportedOperationException();
        }
        @Override
        public boolean isEmpty() {
            throw new UnsupportedOperationException();
        }
        @Override
        public boolean containsValue(Object value) {
            throw new UnsupportedOperationException();
        }
        @Override