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
map.put("failure-mock", (factories, tag, description, config) -> processorFailure);
        IngestService ingestService = createWithProcessors(map);
        final IngestStats initialStats = ingestService.stats();
        assertThat(initialStats.getPipelineStats().size(), equalTo(0));
        assertStats(initialStats.getTotalStats(), 0, 0, 0);
        PutPipelineRequest putRequest = new PutPipelineRequest(
            "_id1",
            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));
        putRequest = new PutPipelineRequest("_id2", new BytesArray("{\"processors\": [{\"mock\" : {}}]}"), XContentType.JSON);
        previousClusterState = clusterState;
        clusterState = executePut(putRequest, clusterState);
        ingestService.applyClusterState(new ClusterChangedEvent("", clusterState, previousClusterState));
        @SuppressWarnings("unchecked")
        final BiConsumer<Integer, Exception> failureHandler = mock(BiConsumer.class);
        @SuppressWarnings("unchecked")
        final BiConsumer<Thread, Exception> completionHandler = mock(BiConsumer.class);
        final IndexRequest indexRequest = new IndexRequest("_index");
        indexRequest.setPipeline("_id1").setFinalPipeline("_none");
        indexRequest.source(randomAlphaOfLength(10), randomAlphaOfLength(10));
        ingestService.executeBulkRequest(1, List.of(indexRequest), indexReq -> {}, failureHandler, completionHandler, Names.WRITE);
        final IngestStats afterFirstRequestStats = ingestService.stats();
        assertThat(afterFirstRequestStats.getPipelineStats().size(), equalTo(2));
        afterFirstRequestStats.getProcessorStats().get("_id1").forEach(p -> assertEquals(p.getName(), "mock:mockTag"));
        afterFirstRequestStats.getProcessorStats().get("_id2").forEach(p -> assertEquals(p.getName(), "mock:mockTag"));