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));
        final IndexRequest indexRequest = new IndexRequest("_index").id("_id")
            .source(Map.of())
            .setPipeline("_id")
            .setFinalPipeline("_none");
        @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(failureHandler, never()).accept(any(), any());
        verify(completionHandler, times(1)).accept(Thread.currentThread(), null);
    }
    public void testDynamicTemplates() throws Exception {
        IngestService ingestService = createWithProcessors(
            Map.of(
                "set",
                (factories, tag, description, config) -> new FakeProcessor(
                    "set",
                    "",
                    "",
                    (ingestDocument) -> ingestDocument.setFieldValue("_dynamic_templates", Map.of("foo", "bar", "foo.bar", "baz"))
                )
            )
        );
        PutPipelineRequest putRequest = new PutPipelineRequest(
            "_id",
            new BytesArray("{\"processors\": [{\"set\" : {}}]}"),
            XContentType.JSON
        );