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
ingestService.executeBulkRequest(1, List.of(indexRequest), indexReq -> {}, failureHandler, completionHandler, Names.WRITE);
        final IngestStats afterThirdRequestStats = ingestService.stats();
        assertThat(afterThirdRequestStats.getPipelineStats().size(), equalTo(2));
        
        assertStats(afterThirdRequestStats.getTotalStats(), 3, 0, 0);
        
        assertPipelineStats(afterThirdRequestStats.getPipelineStats(), "_id1", 2, 0, 0);
        assertPipelineStats(afterThirdRequestStats.getPipelineStats(), "_id2", 1, 0, 0);
        
        
        
        
        assertProcessorStats(0, afterThirdRequestStats, "_id1", 1, 0, 0);
        assertProcessorStats(1, afterThirdRequestStats, "_id1", 1, 0, 0);
        assertProcessorStats(0, afterThirdRequestStats, "_id2", 1, 0, 0);
        
        putRequest = new PutPipelineRequest("_id1", new BytesArray("""
            {"processors": [{"failure-mock" : { "on_failure": [{"mock" : {}}]}}, {"mock" : {}}]}"""), XContentType.JSON);
        previousClusterState = clusterState;
        clusterState = executePut(putRequest, clusterState);
        ingestService.applyClusterState(new ClusterChangedEvent("", clusterState, previousClusterState));
        indexRequest.setPipeline("_id1");
        ingestService.executeBulkRequest(1, List.of(indexRequest), indexReq -> {}, failureHandler, completionHandler, Names.WRITE);
        final IngestStats afterForthRequestStats = ingestService.stats();
        assertThat(afterForthRequestStats.getPipelineStats().size(), equalTo(2));
        
        assertStats(afterForthRequestStats.getTotalStats(), 4, 0, 0);
        
        assertPipelineStats(afterForthRequestStats.getPipelineStats(), "_id1", 3, 0, 0);
        assertPipelineStats(afterForthRequestStats.getPipelineStats(), "_id2", 1, 0, 0);