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 String pipelineId = indexRequest.getPipeline();
                        indexRequest.setPipeline(NOOP_PIPELINE_NAME);
                        final String finalPipelineId = indexRequest.getFinalPipeline();
                        indexRequest.setFinalPipeline(NOOP_PIPELINE_NAME);
                        boolean hasFinalPipeline = true;
                        final List<String> pipelines;
                        if (IngestService.NOOP_PIPELINE_NAME.equals(pipelineId) == false
                            && IngestService.NOOP_PIPELINE_NAME.equals(finalPipelineId) == false) {
                            pipelines = List.of(pipelineId, finalPipelineId);
                        } else if (IngestService.NOOP_PIPELINE_NAME.equals(pipelineId) == false) {
                            pipelines = List.of(pipelineId);
                            hasFinalPipeline = false;
                        } else if (IngestService.NOOP_PIPELINE_NAME.equals(finalPipelineId) == false) {
                            pipelines = List.of(finalPipelineId);
                        } else {
                            i++;
                            continue;
                        }
                        executePipelines(
                            i,
                            pipelines.iterator(),
                            hasFinalPipeline,
                            indexRequest,
                            onDropped,
                            onFailure,
                            refs.acquireListener()
                        );
                        i++;
                    }
                }