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
}
            try {
                Pipeline newPipeline = Pipeline.create(
                    newConfiguration.getId(),
                    newConfiguration.getConfigAsMap(),
                    processorFactories,
                    scriptService
                );
                newPipelines.put(newConfiguration.getId(), new PipelineHolder(newConfiguration, newPipeline));
                if (previous == null) {
                    continue;
                }
                Pipeline oldPipeline = previous.pipeline;
                newPipeline.getMetrics().add(oldPipeline.getMetrics());
                List<Tuple<Processor, IngestMetric>> oldPerProcessMetrics = new ArrayList<>();
                List<Tuple<Processor, IngestMetric>> newPerProcessMetrics = new ArrayList<>();
                getProcessorMetrics(oldPipeline.getCompoundProcessor(), oldPerProcessMetrics);
                getProcessorMetrics(newPipeline.getCompoundProcessor(), newPerProcessMetrics);
                
                
                
                if (newPerProcessMetrics.size() == oldPerProcessMetrics.size()) {
                    Iterator<Tuple<Processor, IngestMetric>> oldMetricsIterator = oldPerProcessMetrics.iterator();
                    for (Tuple<Processor, IngestMetric> compositeMetric : newPerProcessMetrics) {
                        String type = compositeMetric.v1().getType();
                        IngestMetric metric = compositeMetric.v2();
                        if (oldMetricsIterator.hasNext()) {
                            Tuple<Processor, IngestMetric> oldCompositeMetric = oldMetricsIterator.next();
                            String oldType = oldCompositeMetric.v1().getType();
                            IngestMetric oldMetric = oldCompositeMetric.v2();