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/internalClusterTest/java/org/elasticsearch/index/FinalPipelineIT.java
getFinal(parameters, randomBoolean()),
                "request",
                (processorFactories, tag, description, config) -> new AbstractProcessor(tag, description) {
                    @Override
                    public IngestDocument execute(final IngestDocument ingestDocument) throws Exception {
                        ingestDocument.setFieldValue("request", true);
                        return ingestDocument;
                    }
                    @Override
                    public String getType() {
                        return "request";
                    }
                },
                "changing_dest",
                (processorFactories, tag, description, config) -> new AbstractProcessor(tag, description) {
                    @Override
                    public IngestDocument execute(final IngestDocument ingestDocument) throws Exception {
                        ingestDocument.setFieldValue(IngestDocument.Metadata.INDEX.getFieldName(), "target");
                        return ingestDocument;
                    }
                    @Override
                    public String getType() {
                        return "changing_dest";
                    }
                }
            );
        }
        private static Processor.Factory getDefault(Processor.Parameters parameters, boolean async) {
            return (factories, tag, description, config) -> new AbstractProcessor(tag, description) {
                @Override