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/test/java/org/elasticsearch/ingest/IngestServiceTests.java
.setFinalPipeline("_none");
        bulkRequest.add(indexRequest1);
        IndexRequest indexRequest2 = new IndexRequest("_index").id("_id2").source(Map.of()).setPipeline("_id").setFinalPipeline("_none");
        bulkRequest.add(indexRequest2);
        @SuppressWarnings("unchecked")
        final BiConsumer<Integer, Exception> failureHandler = mock(BiConsumer.class);
        @SuppressWarnings("unchecked")
        final BiConsumer<Thread, Exception> completionHandler = mock(BiConsumer.class);
        @SuppressWarnings("unchecked")
        final IntConsumer dropHandler = mock(IntConsumer.class);
        ingestService.executeBulkRequest(
            bulkRequest.numberOfActions(),
            bulkRequest.requests(),
            dropHandler,
            failureHandler,
            completionHandler,
            Names.WRITE
        );
        verify(failureHandler, never()).accept(any(), any());
        verify(completionHandler, times(1)).accept(Thread.currentThread(), null);
        verify(dropHandler, times(1)).accept(1);
    }
    public void testIngestClusterStateListeners_orderOfExecution() {
        final AtomicInteger counter = new AtomicInteger(0);
        
        Consumer<ClusterState> ingestClusterStateListener = clusterState -> { assertThat(counter.compareAndSet(0, 1), is(true)); };
        
        IngestPlugin testPlugin = new IngestPlugin() {
            @Override
            public Map<String, Processor.Factory> getProcessors(Processor.Parameters parameters) {