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
DiscoveryNode node1 = new DiscoveryNode("_node_id1", buildNewFakeTransportAddress(), Map.of(), Set.of(), Version.CURRENT);
        DiscoveryNode node2 = new DiscoveryNode("_node_id2", buildNewFakeTransportAddress(), Map.of(), Set.of(), Version.CURRENT);
        Map<DiscoveryNode, IngestInfo> ingestInfos = new HashMap<>();
        ingestInfos.put(node1, new IngestInfo(List.of(new ProcessorInfo("set"), new ProcessorInfo("remove"))));
        ingestInfos.put(node2, new IngestInfo(List.of(new ProcessorInfo("set"))));
        ElasticsearchParseException e = expectThrows(
            ElasticsearchParseException.class,
            () -> ingestService.validatePipeline(ingestInfos, putRequest.getId(), pipelineConfig)
        );
        assertEquals("Processor type [remove] is not installed on node [" + node2 + "]", e.getMessage());
        assertEquals("remove", e.getMetadata("es.processor_type").get(0));
        assertEquals("tag2", e.getMetadata("es.processor_tag").get(0));
        var pipelineConfig2 = XContentHelper.convertToMap(putRequest.getSource(), false, putRequest.getXContentType()).v2();
        ingestInfos.put(node2, new IngestInfo(List.of(new ProcessorInfo("set"), new ProcessorInfo("remove"))));
        ingestService.validatePipeline(ingestInfos, putRequest.getId(), pipelineConfig2);
    }
    public void testExecuteIndexPipelineExistsButFailedParsing() {
        IngestService ingestService = createWithProcessors(
            Map.of("mock", (factories, tag, description, config) -> new AbstractProcessor("mock", "description") {
                @Override
                public IngestDocument execute(IngestDocument ingestDocument) {
                    throw new IllegalStateException("error");
                }
                @Override
                public String getType() {
                    return null;
                }
            })
        );
        ClusterState clusterState = ClusterState.builder(new ClusterName("_name")).build();