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
var pipelineId = randomAlphaOfLength(5);
        final Integer existingVersion = randomInt();
        var pipelineString = "{\"version\": " + existingVersion + ", \"processors\": []}";
        var existingPipeline = new PipelineConfiguration(pipelineId, new BytesArray(pipelineString), XContentType.JSON);
        var clusterState = ClusterState.builder(new ClusterName("test"))
            .metadata(Metadata.builder().putCustom(IngestMetadata.TYPE, new IngestMetadata(Map.of(pipelineId, existingPipeline))).build())
            .build();
        var updatedPipelineString = "{\"processors\": []}";
        var request = new PutPipelineRequest(pipelineId, new BytesArray(updatedPipelineString), XContentType.JSON, existingVersion);
        var updatedState = executePut(request, clusterState);
        var updatedConfig = ((IngestMetadata) updatedState.metadata().custom(IngestMetadata.TYPE)).getPipelines().get(pipelineId);
        assertThat(updatedConfig, notNullValue());
        assertThat(updatedConfig.getVersion(), equalTo(existingVersion + 1));
    }
    public void testResolvePipelinesWithNonePipeline() {
        
        {
            Metadata metadata = Metadata.builder().build();
            IndexRequest indexRequest = new IndexRequest("idx").setPipeline(IngestService.NOOP_PIPELINE_NAME);
            boolean result = IngestService.resolvePipelines(indexRequest, indexRequest, metadata);
            assertThat(result, is(false));
            assertThat(indexRequest.isPipelineResolved(), is(true));
            assertThat(indexRequest.getPipeline(), equalTo(IngestService.NOOP_PIPELINE_NAME));
        }
        
        {
            IndexMetadata.Builder builder = IndexMetadata.builder("idx")
                .settings(settings(Version.CURRENT).put(IndexSettings.DEFAULT_PIPELINE.getKey(), IngestService.NOOP_PIPELINE_NAME))
                .numberOfShards(1)
                .numberOfReplicas(0);