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
numRequest,
            bulkRequest.requests(),
            indexReq -> {},
            requestItemErrorHandler,
            completionHandler,
            Names.WRITE
        );
        verify(requestItemErrorHandler, times(numIndexRequests)).accept(anyInt(), argThat(e -> e.getCause().equals(error)));
        verify(completionHandler, times(1)).accept(Thread.currentThread(), null);
    }
    public void testBulkRequestExecution() throws Exception {
        BulkRequest bulkRequest = new BulkRequest();
        String pipelineId = "_id";
        
        XContentType xContentType = randomFrom(
            Arrays.stream(XContentType.values()).filter(t -> Requests.INDEX_CONTENT_TYPE.equals(t) == false).toList()
        );
        logger.info("Using [{}], not randomly determined default [{}]", xContentType, Requests.INDEX_CONTENT_TYPE);
        int numRequest = scaledRandomIntBetween(8, 64);
        for (int i = 0; i < numRequest; i++) {
            IndexRequest indexRequest = new IndexRequest("_index").id("_id").setPipeline(pipelineId).setFinalPipeline("_none");
            indexRequest.source(xContentType, "field1", "value1");
            bulkRequest.add(indexRequest);
        }
        final Processor processor = mock(Processor.class);
        when(processor.getType()).thenReturn("mock");
        when(processor.getTag()).thenReturn("mockTag");
        doAnswer(args -> {
            @SuppressWarnings("unchecked")
            BiConsumer<IngestDocument, Exception> handler = (BiConsumer) args.getArguments()[1];