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
public Object put(String key, Object value) {
            throw new UnsupportedOperationException();
        }
        @Override
        public Object remove(Object key) {
            throw new UnsupportedOperationException();
        }
        @Override
        public void putAll(Map<? extends String, ?> m) {
            throw new UnsupportedOperationException();
        }
        @Override
        public void clear() {
            throw new UnsupportedOperationException();
        }
        @Override
        public Set<String> keySet() {
            throw new UnsupportedOperationException();
        }
        @Override
        public Collection<Object> values() {
            throw new UnsupportedOperationException();
        }
        @Override
        public Set<Entry<String, Object>> entrySet() {
            throw new UnsupportedOperationException();
        }
    }
}