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
pipelineToReturn[0] = ingestService.getPipeline("_id3");
        {
            final IngestStats ingestStats = ingestService.stats();
            assertThat(ingestStats.getPipelineStats().size(), equalTo(3));
            
            assertStats(ingestStats.getTotalStats(), 0, 0, 0);
            
            assertPipelineStats(ingestStats.getPipelineStats(), "_id1", 0, 0, 0);
            assertPipelineStats(ingestStats.getPipelineStats(), "_id2", 0, 0, 0);
            assertPipelineStats(ingestStats.getPipelineStats(), "_id3", 0, 0, 0);
            
            assertProcessorStats(0, ingestStats, "_id1", 0, 0, 0);
            assertProcessorStats(0, ingestStats, "_id2", 0, 0, 0);
            assertProcessorStats(0, ingestStats, "_id3", 0, 0, 0);
        }
        
        final IndexRequest indexRequest = new IndexRequest("_index");
        indexRequest.setPipeline("_id1").setFinalPipeline("_id2");
        indexRequest.source(randomAlphaOfLength(10), randomAlphaOfLength(10));
        ingestService.executeBulkRequest(1, List.of(indexRequest), indexReq -> {}, (integer, e) -> {}, (thread, e) -> {}, Names.WRITE);
        {
            final IngestStats ingestStats = ingestService.stats();
            assertThat(ingestStats.getPipelineStats().size(), equalTo(3));
            
            
            
            
            assertPipelineStats(ingestStats.getPipelineStats(), "_id1", 1, 0, 0);
            assertPipelineStats(ingestStats.getPipelineStats(), "_id2", 1, 0, 0);