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/internalClusterTest/java/org/elasticsearch/index/FinalPipelineIT.java
@Override
        public Collection<Object> createComponents(
            final Client client,
            final ClusterService clusterService,
            final ThreadPool threadPool,
            final ResourceWatcherService resourceWatcherService,
            final ScriptService scriptService,
            final NamedXContentRegistry xContentRegistry,
            final Environment environment,
            final NodeEnvironment nodeEnvironment,
            final NamedWriteableRegistry namedWriteableRegistry,
            final IndexNameExpressionResolver expressionResolver,
            final Supplier<RepositoriesService> repositoriesServiceSupplier,
            Tracer tracer,
            AllocationService allocationService
        ) {
            return List.of();
        }
        @Override
        public Map<String, Processor.Factory> getProcessors(Processor.Parameters parameters) {
            return Map.of(
                "default",
                getDefault(parameters, randomBoolean()),
                "final",