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
.metadata(Metadata.builder().putCustom(IngestMetadata.TYPE, ingestMetadata))
            .build();
        ingestService.applyClusterState(new ClusterChangedEvent("", clusterState, previousClusterState));
        assertThat(ingestService.getPipeline("p1"), notNullValue());
        ClusterState finalClusterState = clusterState;
        assertThat(
            expectThrows(ResourceNotFoundException.class, () -> executeFailingDelete(new DeletePipelineRequest("z*"), finalClusterState))
                .getMessage(),
            equalTo("pipeline [z*] is missing")
        );
    }
    public void testDeleteWithIndexUsePipeline() {
        IngestService ingestService = createWithProcessors();
        PipelineConfiguration config = new PipelineConfiguration("_id", new BytesArray("""
            {"processors": [{"set" : {"field": "_field", "value": "_value"}}]}"""), XContentType.JSON);
        IngestMetadata ingestMetadata = new IngestMetadata(Map.of("_id", config));
        Metadata.Builder builder = Metadata.builder();
        for (int i = 0; i < randomIntBetween(2, 10); i++) {
            builder.put(
                IndexMetadata.builder("test" + i).settings(settings(Version.CURRENT)).numberOfShards(1).numberOfReplicas(1).build(),
                true
            );
        }
        builder.putCustom(IngestMetadata.TYPE, ingestMetadata);
        Metadata metadata = builder.build();
        ClusterState clusterState = ClusterState.builder(new ClusterName("_name")).build();
        ClusterState previousClusterState = clusterState;
        clusterState = ClusterState.builder(clusterState).metadata(metadata).build();
        ingestService.applyClusterState(new ClusterChangedEvent("", clusterState, previousClusterState));
        assertThat(ingestService.getPipeline("_id"), notNullValue());