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
String id = "_id";
        PutPipelineRequest putRequest = new PutPipelineRequest(
            id,
            new BytesArray("{\"processors\": [{\"mock\" : {}}]}"),
            XContentType.JSON
        );
        ClusterState previousClusterState = clusterState;
        clusterState = executePut(putRequest, clusterState);
        ingestService.applyClusterState(new ClusterChangedEvent("", clusterState, previousClusterState));
        final SetOnce<Boolean> failure = new SetOnce<>();
        BulkRequest bulkRequest = new BulkRequest();
        final IndexRequest indexRequest1 = new IndexRequest("_index").id("_id1")
            .source(Map.of())
            .setPipeline("_none")
            .setFinalPipeline("_none");
        bulkRequest.add(indexRequest1);
        IndexRequest indexRequest2 = new IndexRequest("_index").id("_id2").source(Map.of()).setPipeline(id).setFinalPipeline("_none");
        bulkRequest.add(indexRequest2);
        final BiConsumer<Integer, Exception> failureHandler = (slot, e) -> {
            assertThat(e.getCause(), instanceOf(IllegalStateException.class));
            assertThat(e.getCause().getMessage(), equalTo("error"));
            failure.set(true);
            assertThat(slot, equalTo(1));
        };
        @SuppressWarnings("unchecked")
        final BiConsumer<Thread, Exception> completionHandler = mock(BiConsumer.class);
        ingestService.executeBulkRequest(
            bulkRequest.numberOfActions(),
            bulkRequest.requests(),
            indexReq -> {},