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
}
    public void testExecuteFailureWithNestedOnFailure() throws Exception {
        final Processor processor = mock(Processor.class);
        when(processor.isAsync()).thenReturn(true);
        final Processor onFailureProcessor = mock(Processor.class);
        when(onFailureProcessor.isAsync()).thenReturn(true);
        final Processor onFailureOnFailureProcessor = mock(Processor.class);
        when(onFailureOnFailureProcessor.isAsync()).thenReturn(true);
        final List<Processor> processors = List.of(onFailureProcessor);
        final List<Processor> onFailureProcessors = List.of(onFailureOnFailureProcessor);
        final CompoundProcessor compoundProcessor = new CompoundProcessor(
            false,
            List.of(processor),
            List.of(new CompoundProcessor(false, processors, onFailureProcessors))
        );
        IngestService ingestService = createWithProcessors(Map.of("mock", (factories, tag, description, config) -> compoundProcessor));
        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 IndexRequest indexRequest = new IndexRequest("_index").id("_id")
            .source(Map.of())
            .setPipeline("_id")
            .setFinalPipeline("_none");
        doThrow(new RuntimeException()).when(onFailureOnFailureProcessor)