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
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 testExecutePropagateAllMetadataUpdates() throws Exception {
        final CompoundProcessor processor = mockCompoundProcessor();
        IngestService ingestService = createWithProcessors(Map.of("mock", (factories, tag, description, config) -> processor));
        PutPipelineRequest putRequest = new PutPipelineRequest(
            "_id",
            new BytesArray("{\"processors\": [{\"mock\" : {}}]}"),
            XContentType.JSON
        );
        ClusterState clusterState = ClusterState.builder(new ClusterName("_name")).build(); 
        ClusterState previousClusterState = clusterState;
        clusterState = executePut(putRequest, clusterState);
        ingestService.applyClusterState(new ClusterChangedEvent("", clusterState, previousClusterState));
        final long newVersion = randomLong();
        final String versionType = randomFrom("internal", "external", "external_gt", "external_gte");
        final long ifSeqNo = randomNonNegativeLong();
        final long ifPrimaryTerm = randomNonNegativeLong();
        doAnswer((InvocationOnMock invocationOnMock) -> {
            IngestDocument ingestDocument = (IngestDocument) invocationOnMock.getArguments()[0];
            for (IngestDocument.Metadata metadata : IngestDocument.Metadata.values()) {
                if (metadata == IngestDocument.Metadata.VERSION) {
                    ingestDocument.setFieldValue(metadata.getFieldName(), newVersion);
                } else if (metadata == IngestDocument.Metadata.VERSION_TYPE) {
                    ingestDocument.setFieldValue(metadata.getFieldName(), versionType);
                } else if (metadata == IngestDocument.Metadata.IF_SEQ_NO) {
                    ingestDocument.setFieldValue(metadata.getFieldName(), ifSeqNo);