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