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
BiConsumer<IngestDocument, Exception> handler = (BiConsumer) args.getArguments()[1];
            handler.accept(ingestDocument, null);
            return null;
        }).when(onFailureProcessor).execute(eqIndexTypeId(Map.of()), any());
        final CompoundProcessor compoundProcessor = new CompoundProcessor(
            false,
            List.of(processor),
            List.of(new CompoundProcessor(onFailureProcessor))
        );
        IngestService ingestService = createWithProcessors(Map.of("mock", (factories, tag, description, config) -> compoundProcessor));
        PutPipelineRequest putRequest = new PutPipelineRequest(
            "_id",
            new BytesArray("{\"processors\": [{\"mock\" : {}}]}"),
            XContentType.JSON
        );
        ClusterState clusterState = ClusterState.builder(new ClusterName("_name")).build(); 
        ClusterState previousClusterState = clusterState;
        clusterState = executePut(putRequest, clusterState);
        ingestService.applyClusterState(new ClusterChangedEvent("", clusterState, previousClusterState));
        final IndexRequest indexRequest = new IndexRequest("_index").id("_id")
            .source(Map.of())
            .setPipeline("_id")
            .setFinalPipeline("_none");
        @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(failureHandler, never()).accept(eq(0), any(IngestProcessorException.class));
        verify(completionHandler, times(1)).accept(Thread.currentThread(), null);