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
failureHandler,
            completionHandler,
            Names.WRITE
        );
        assertTrue(failure.get());
        verify(completionHandler, times(1)).accept(Thread.currentThread(), null);
    }
    public void testExecuteBulkPipelineDoesNotExist() {
        IngestService ingestService = createWithProcessors(
            Map.of("mock", (factories, tag, description, config) -> mockCompoundProcessor())
        );
        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));
        BulkRequest bulkRequest = new BulkRequest();
        IndexRequest indexRequest1 = new IndexRequest("_index").id("_id1").source(Map.of()).setPipeline("_none").setFinalPipeline("_none");
        bulkRequest.add(indexRequest1);
        IndexRequest indexRequest2 = new IndexRequest("_index").id("_id2").source(Map.of()).setPipeline("_id").setFinalPipeline("_none");
        bulkRequest.add(indexRequest2);
        IndexRequest indexRequest3 = new IndexRequest("_index").id("_id3")
            .source(Map.of())
            .setPipeline("does_not_exist")
            .setFinalPipeline("_none");
        bulkRequest.add(indexRequest3);