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
handler.accept(RandomDocumentPicks.randomIngestDocument(random()), null);
            return null;
        }).when(processor).execute(any(), any());
        Map<String, Processor.Factory> map = Maps.newMapWithExpectedSize(2);
        map.put("mock", (factories, tag, description, config) -> processor);
        IngestService ingestService = createWithProcessors(map);
        PutPipelineRequest putRequest = new PutPipelineRequest("_id", new BytesArray("""
            {"processors": [{"mock": {}}], "description": "_description"}"""), XContentType.JSON);
        ClusterState clusterState = ClusterState.builder(new ClusterName("_name")).build();
        ClusterState previousClusterState = clusterState;
        clusterState = executePut(putRequest, clusterState);
        ingestService.applyClusterState(new ClusterChangedEvent("", clusterState, previousClusterState));
        @SuppressWarnings("unchecked")
        BiConsumer<Integer, Exception> requestItemErrorHandler = mock(BiConsumer.class);
        @SuppressWarnings("unchecked")
        final BiConsumer<Thread, Exception> completionHandler = mock(BiConsumer.class);
        ingestService.executeBulkRequest(
            numRequest,
            bulkRequest.requests(),
            indexReq -> {},
            requestItemErrorHandler,
            completionHandler,
            Names.WRITE
        );
        verify(requestItemErrorHandler, never()).accept(any(), any());
        verify(completionHandler, times(1)).accept(Thread.currentThread(), null);
        for (DocWriteRequest<?> docWriteRequest : bulkRequest.requests()) {
            IndexRequest indexRequest = TransportBulkAction.getIndexWriteRequest(docWriteRequest);
            assertThat(indexRequest, notNullValue());
            assertThat(indexRequest.getContentType(), equalTo(xContentType.canonical()));