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
* has been forwarded by a non-ingest coordinating node to an ingest node. In this case, the coordinating node will have
             * already resolved the pipeline for this request. It is important that we are able to distinguish this situation as we
             * can not double-resolve the pipeline because we will not be able to distinguish the case of the pipeline having been
             * set from a request pipeline parameter versus having been set by the resolution. We need to be able to distinguish
             * these cases as we need to reject the request if the pipeline was set by a required pipeline and there is a request
             * pipeline parameter too.
             */
            indexRequest.isPipelineResolved(true);
        }
        
        return NOOP_PIPELINE_NAME.equals(indexRequest.getPipeline()) == false
            || NOOP_PIPELINE_NAME.equals(indexRequest.getFinalPipeline()) == false;
    }
    public ClusterService getClusterService() {
        return clusterService;
    }
    public ScriptService getScriptService() {
        return scriptService;
    }
    /**
     * Deletes the pipeline specified by id in the request.
     */
    public void delete(DeletePipelineRequest request, ActionListener<AcknowledgedResponse> listener) {
        clusterService.submitStateUpdateTask(
            "delete-pipeline-" + request.getId(),
            new DeletePipelineClusterStateUpdateTask(listener, request),
            ClusterStateTaskConfig.build(Priority.NORMAL, request.masterNodeTimeout()),
            PIPELINE_TASK_EXECUTOR
        );
    }