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
ingestService.applyClusterState(new ClusterChangedEvent("", clusterState, previousClusterState));
        final IndexRequest indexRequest = new IndexRequest("_index").id("_id")
            .source(Map.of())
            .setPipeline("_id1")
            .setFinalPipeline("_id2");
        doThrow(new RuntimeException()).when(processor)
            .execute(eqIndexTypeId(indexRequest.version(), indexRequest.versionType(), Map.of()), any());
        @SuppressWarnings("unchecked")
        final BiConsumer<Integer, Exception> failureHandler = mock(BiConsumer.class);
        @SuppressWarnings("unchecked")
        final BiConsumer<Thread, Exception> completionHandler = mock(BiConsumer.class);
        ingestService.executeBulkRequest(1, List.of(indexRequest), indexReq -> {}, failureHandler, completionHandler, Names.WRITE);
        verify(processor).execute(eqIndexTypeId(indexRequest.version(), indexRequest.versionType(), Map.of()), any());
        verify(failureHandler, times(1)).accept(eq(0), any(RuntimeException.class));
        verify(completionHandler, times(1)).accept(Thread.currentThread(), null);
    }
    public void testExecuteSuccessWithOnFailure() throws Exception {
        final Processor processor = mock(Processor.class);
        when(processor.getType()).thenReturn("mock_processor_type");
        when(processor.getTag()).thenReturn("mock_processor_tag");
        doAnswer(args -> {
            @SuppressWarnings("unchecked")
            BiConsumer<IngestDocument, Exception> handler = (BiConsumer) args.getArguments()[1];
            handler.accept(null, new RuntimeException());
            return null;
        }).when(processor).execute(eqIndexTypeId(Map.of()), any());
        final Processor onFailureProcessor = mock(Processor.class);
        doAnswer(args -> {
            IngestDocument ingestDocument = (IngestDocument) args.getArguments()[0];
            @SuppressWarnings("unchecked")