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
.execute(eqIndexTypeId(indexRequest.version(), indexRequest.versionType(), Map.of()), any());
        doThrow(new RuntimeException()).when(onFailureProcessor)
            .execute(eqIndexTypeId(indexRequest.version(), indexRequest.versionType(), Map.of()), any());
        doThrow(new RuntimeException()).when(processor)
            .execute(eqIndexTypeId(indexRequest.version(), indexRequest.versionType(), Map.of()), any());
        @SuppressWarnings("unchecked")
        final BiConsumer<Integer, Exception> failureHandler = mock(BiConsumer.class);
        @SuppressWarnings("unchecked")
        final BiConsumer<Thread, Exception> completionHandler = mock(BiConsumer.class);
        ingestService.executeBulkRequest(1, List.of(indexRequest), indexReq -> {}, failureHandler, completionHandler, Names.WRITE);
        verify(processor).execute(eqIndexTypeId(indexRequest.version(), indexRequest.versionType(), Map.of()), any());
        verify(failureHandler, times(1)).accept(eq(0), any(RuntimeException.class));
        verify(completionHandler, times(1)).accept(Thread.currentThread(), null);
    }
    public void testBulkRequestExecutionWithFailures() throws Exception {
        BulkRequest bulkRequest = new BulkRequest();
        String pipelineId = "_id";
        int numRequest = scaledRandomIntBetween(8, 64);
        int numIndexRequests = 0;
        for (int i = 0; i < numRequest; i++) {
            DocWriteRequest<?> request;
            if (randomBoolean()) {
                if (randomBoolean()) {
                    request = new DeleteRequest("_index", "_id");
                } else {
                    request = new UpdateRequest("_index", "_id");
                }
            } else {
                IndexRequest indexRequest = new IndexRequest("_index").id("_id").setPipeline(pipelineId).setFinalPipeline("_none");
                indexRequest.source(Requests.INDEX_CONTENT_TYPE, "field1", "value1");