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
} else if (metadata == IngestDocument.Metadata.IF_PRIMARY_TERM) {
                    ingestDocument.setFieldValue(metadata.getFieldName(), ifPrimaryTerm);
                } else if (metadata == IngestDocument.Metadata.DYNAMIC_TEMPLATES) {
                    ingestDocument.setFieldValue(metadata.getFieldName(), Map.of("foo", "bar"));
                } else if (metadata == IngestDocument.Metadata.TYPE) {
                    
                } else {
                    ingestDocument.setFieldValue(metadata.getFieldName(), "update" + metadata.getFieldName());
                }
            }
            @SuppressWarnings("unchecked")
            BiConsumer<IngestDocument, Exception> handler = (BiConsumer<IngestDocument, Exception>) invocationOnMock.getArguments()[1];
            handler.accept(ingestDocument, null);
            return null;
        }).when(processor).execute(any(), any());
        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(processor).execute(any(), any());
        verify(failureHandler, never()).accept(any(), any());
        verify(completionHandler, times(1)).accept(Thread.currentThread(), null);
        assertThat(indexRequest.index(), equalTo("update_index"));
        assertThat(indexRequest.id(), equalTo("update_id"));
        assertThat(indexRequest.routing(), equalTo("update_routing"));