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<String, IngestService.PipelineHolder> pipelines = ingestService.pipelines();
        ingestService.innerUpdatePipelines(ingestMetadata);
        assertThat(ingestService.pipelines(), sameInstance(pipelines));
    }
    public void testDelete() {
        IngestService ingestService = createWithProcessors();
        PipelineConfiguration config = new PipelineConfiguration("_id", new BytesArray("""
            {"processors": [{"set" : {"field": "_field", "value": "_value"}}]}"""), XContentType.JSON);
        IngestMetadata ingestMetadata = new IngestMetadata(Map.of("_id", config));
        ClusterState clusterState = ClusterState.builder(new ClusterName("_name")).build();
        ClusterState previousClusterState = clusterState;
        clusterState = ClusterState.builder(clusterState)
            .metadata(Metadata.builder().putCustom(IngestMetadata.TYPE, ingestMetadata))
            .build();
        ingestService.applyClusterState(new ClusterChangedEvent("", clusterState, previousClusterState));
        assertThat(ingestService.getPipeline("_id"), notNullValue());
        
        DeletePipelineRequest deleteRequest = new DeletePipelineRequest("_id");
        previousClusterState = clusterState;
        clusterState = executeDelete(deleteRequest, clusterState);
        ingestService.applyClusterState(new ClusterChangedEvent("", clusterState, previousClusterState));
        assertThat(ingestService.getPipeline("_id"), nullValue());
        
        ClusterState finalClusterState = clusterState;
        assertThat(
            expectThrows(ResourceNotFoundException.class, () -> executeFailingDelete(deleteRequest, finalClusterState)).getMessage(),
            equalTo("pipeline [_id] is missing")
        );
    }
    public void testValidateNoIngestInfo() throws Exception {