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
return processorMetrics;
    }
    /**
     * Used in this class and externally by the {@link org.elasticsearch.action.ingest.ReservedPipelineAction}
     */
    public static class PutPipelineClusterStateUpdateTask extends PipelineClusterStateUpdateTask {
        private final PutPipelineRequest request;
        PutPipelineClusterStateUpdateTask(ActionListener<AcknowledgedResponse> listener, PutPipelineRequest request) {
            super(listener);
            this.request = request;
        }
        /**
         * Used by {@link org.elasticsearch.action.ingest.ReservedPipelineAction}
         */
        public PutPipelineClusterStateUpdateTask(PutPipelineRequest request) {
            this(null, request);
        }
        @Override
        public IngestMetadata execute(IngestMetadata currentIngestMetadata, Collection<IndexMetadata> allIndexMetadata) {
            BytesReference pipelineSource = request.getSource();
            if (request.getVersion() != null) {
                var currentPipeline = currentIngestMetadata != null ? currentIngestMetadata.getPipelines().get(request.getId()) : null;
                if (currentPipeline == null) {
                    throw new IllegalArgumentException(
                        String.format(
                            Locale.ROOT,
                            "version conflict, required version [%s] for pipeline [%s] but no pipeline was found",
                            request.getVersion(),
                            request.getId()
                        )