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
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.env.Environment;
import org.elasticsearch.env.NodeEnvironment;
import org.elasticsearch.ingest.AbstractProcessor;
import org.elasticsearch.ingest.IngestDocument;
import org.elasticsearch.ingest.PipelineConfiguration;
import org.elasticsearch.ingest.Processor;
import org.elasticsearch.plugins.IngestPlugin;
import org.elasticsearch.plugins.Plugin;
import org.elasticsearch.repositories.RepositoriesService;
import org.elasticsearch.rest.RestStatus;
import org.elasticsearch.script.ScriptService;
import org.elasticsearch.test.ESIntegTestCase;
import org.elasticsearch.threadpool.ThreadPool;
import org.elasticsearch.tracing.Tracer;
import org.elasticsearch.watcher.ResourceWatcherService;
import org.elasticsearch.xcontent.NamedXContentRegistry;
import org.elasticsearch.xcontent.XContentType;
import org.junit.After;
import java.io.IOException;
import java.util.Collection;
import java.util.List;
import java.util.Map;
import java.util.function.BiConsumer;
import java.util.function.Supplier;
import static org.hamcrest.Matchers.containsString;
import static org.hamcrest.Matchers.equalTo;
import static org.hamcrest.Matchers.hasKey;
import static org.hamcrest.Matchers.hasToString;
public class FinalPipelineIT extends ESIntegTestCase {