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/main/java/org/elasticsearch/ingest/IngestService.java
final ActionListener<AcknowledgedResponse> listener;
        PipelineClusterStateUpdateTask(ActionListener<AcknowledgedResponse> listener) {
            this.listener = listener;
        }
        public abstract IngestMetadata execute(IngestMetadata currentIngestMetadata, Collection<IndexMetadata> allIndexMetadata);
        @Override
        public void onFailure(Exception e) {
            listener.onFailure(e);
        }
    }
    public IngestService(
        ClusterService clusterService,
        ThreadPool threadPool,
        Environment env,
        ScriptService scriptService,
        AnalysisRegistry analysisRegistry,
        List<IngestPlugin> ingestPlugins,
        Client client
    ) {
        this.clusterService = clusterService;
        this.scriptService = scriptService;
        this.processorFactories = processorFactories(
            ingestPlugins,
            new Processor.Parameters(
                env,
                scriptService,
                analysisRegistry,