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
var builder = XContentBuilder.builder(request.getXContentType().xContent()).map(pipelineConfig);
                        pipelineSource = BytesReference.bytes(builder);
                    } catch (IOException e) {
                        throw new IllegalStateException(e);
                    }
                }
            }
            Map<String, PipelineConfiguration> pipelines;
            if (currentIngestMetadata != null) {
                pipelines = new HashMap<>(currentIngestMetadata.getPipelines());
            } else {
                pipelines = new HashMap<>();
            }
            pipelines.put(request.getId(), new PipelineConfiguration(request.getId(), pipelineSource, request.getXContentType()));
            return new IngestMetadata(pipelines);
        }
    }
    void validatePipeline(Map<DiscoveryNode, IngestInfo> ingestInfos, String pipelineId, Map<String, Object> pipelineConfig)
        throws Exception {
        if (ingestInfos.isEmpty()) {
            throw new IllegalStateException("Ingest info is empty");
        }
        Pipeline pipeline = Pipeline.create(pipelineId, pipelineConfig, processorFactories, scriptService);
        List<Exception> exceptions = new ArrayList<>();
        for (Processor processor : pipeline.flattenAllProcessors()) {
            for (Map.Entry<DiscoveryNode, IngestInfo> entry : ingestInfos.entrySet()) {
                String type = processor.getType();
                if (entry.getValue().containsProcessor(type) == false && ConditionalProcessor.TYPE.equals(type) == false) {
                    String message = "Processor type [" + processor.getType() + "] is not installed on node [" + entry.getKey() + "]";
                    exceptions.add(ConfigurationUtils.newConfigurationException(processor.getType(), processor.getTag(), null, message));