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
DeletePipelineRequest deleteRequest = new DeletePipelineRequest("_id");
        {
            
            IndexMetadata indexMetadata = IndexMetadata.builder("pipeline-index")
                .settings(settings(Version.CURRENT).put(IndexSettings.DEFAULT_PIPELINE.getKey(), "_id"))
                .numberOfShards(1)
                .numberOfReplicas(1)
                .build();
            ClusterState finalClusterState = ClusterState.builder(clusterState)
                .metadata(Metadata.builder(metadata).put(indexMetadata, true))
                .build();
            IllegalArgumentException e = expectThrows(
                IllegalArgumentException.class,
                () -> executeFailingDelete(deleteRequest, finalClusterState)
            );
            assertThat(e.getMessage(), containsString("default pipeline for 1 index(es) including [pipeline-index]"));
        }
        {
            
            IndexMetadata indexMetadata = IndexMetadata.builder("pipeline-index")
                .settings(settings(Version.CURRENT).put(IndexSettings.FINAL_PIPELINE.getKey(), "_id"))
                .numberOfShards(1)
                .numberOfReplicas(1)
                .build();
            ClusterState finalClusterState = ClusterState.builder(clusterState)
                .metadata(Metadata.builder(metadata).put(indexMetadata, true))
                .build();
            IllegalArgumentException e = expectThrows(
                IllegalArgumentException.class,
                () -> executeFailingDelete(deleteRequest, finalClusterState)