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
import org.elasticsearch.cluster.ClusterState;
import org.elasticsearch.cluster.metadata.AliasMetadata;
import org.elasticsearch.cluster.metadata.DataStream.TimestampField;
import org.elasticsearch.cluster.metadata.IndexMetadata;
import org.elasticsearch.cluster.metadata.IndexTemplateMetadata;
import org.elasticsearch.cluster.metadata.Metadata;
import org.elasticsearch.cluster.node.DiscoveryNode;
import org.elasticsearch.cluster.service.ClusterService;
import org.elasticsearch.cluster.service.ClusterStateTaskExecutorUtils;
import org.elasticsearch.common.bytes.BytesArray;
import org.elasticsearch.common.logging.Loggers;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.time.DateFormatter;
import org.elasticsearch.common.util.Maps;
import org.elasticsearch.common.util.concurrent.EsExecutors;
import org.elasticsearch.common.xcontent.XContentHelper;
import org.elasticsearch.core.Strings;
import org.elasticsearch.core.Tuple;
import org.elasticsearch.index.IndexSettings;
import org.elasticsearch.index.VersionType;
import org.elasticsearch.plugins.IngestPlugin;
import org.elasticsearch.script.MockScriptEngine;
import org.elasticsearch.script.Script;
import org.elasticsearch.script.ScriptModule;
import org.elasticsearch.script.ScriptService;
import org.elasticsearch.script.ScriptType;
import org.elasticsearch.script.TemplateScript;
import org.elasticsearch.test.ESTestCase;
import org.elasticsearch.test.MockLogAppender;
import org.elasticsearch.threadpool.ThreadPool;