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
ingestService.applyClusterState(new ClusterChangedEvent("", clusterState, previousClusterState));
        assertThat(ingestService.pipelines().size(), is(0));
        PipelineConfiguration pipeline = new PipelineConfiguration("_id", new BytesArray("""
            {"processors": [{"set" : {"field": "_field", "value": "_value"}}]}"""), XContentType.JSON);
        IngestMetadata ingestMetadata = new IngestMetadata(Map.of("_id", pipeline));
        clusterState = ClusterState.builder(clusterState)
            .metadata(Metadata.builder().putCustom(IngestMetadata.TYPE, ingestMetadata))
            .build();
        ingestService.applyClusterState(new ClusterChangedEvent("", clusterState, previousClusterState));
        assertThat(ingestService.pipelines().size(), is(1));
        assertThat(ingestService.pipelines().get("_id").pipeline.getId(), equalTo("_id"));
        assertThat(ingestService.pipelines().get("_id").pipeline.getDescription(), nullValue());
        assertThat(ingestService.pipelines().get("_id").pipeline.getProcessors().size(), equalTo(1));
        assertThat(ingestService.pipelines().get("_id").pipeline.getProcessors().get(0).getType(), equalTo("set"));
    }
    public void testInnerUpdatePipelines() {
        IngestService ingestService = createWithProcessors();
        assertThat(ingestService.pipelines().size(), is(0));
        PipelineConfiguration pipeline1 = new PipelineConfiguration("_id1", new BytesArray("{\"processors\": []}"), XContentType.JSON);
        IngestMetadata ingestMetadata = new IngestMetadata(Map.of("_id1", pipeline1));
        ingestService.innerUpdatePipelines(ingestMetadata);
        assertThat(ingestService.pipelines().size(), is(1));
        assertThat(ingestService.pipelines().get("_id1").pipeline.getId(), equalTo("_id1"));
        assertThat(ingestService.pipelines().get("_id1").pipeline.getProcessors().size(), equalTo(0));
        PipelineConfiguration pipeline2 = new PipelineConfiguration("_id2", new BytesArray("{\"processors\": []}"), XContentType.JSON);
        ingestMetadata = new IngestMetadata(Map.of("_id1", pipeline1, "_id2", pipeline2));
        ingestService.innerUpdatePipelines(ingestMetadata);
        assertThat(ingestService.pipelines().size(), is(2));
        assertThat(ingestService.pipelines().get("_id1").pipeline.getId(), equalTo("_id1"));
        assertThat(ingestService.pipelines().get("_id1").pipeline.getProcessors().size(), equalTo(0));