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/main/java/org/elasticsearch/ingest/IngestService.java
import org.elasticsearch.action.support.RefCountingRunnable;
import org.elasticsearch.action.support.master.AcknowledgedResponse;
import org.elasticsearch.client.internal.Client;
import org.elasticsearch.cluster.ClusterChangedEvent;
import org.elasticsearch.cluster.ClusterState;
import org.elasticsearch.cluster.ClusterStateApplier;
import org.elasticsearch.cluster.ClusterStateTaskConfig;
import org.elasticsearch.cluster.ClusterStateTaskExecutor;
import org.elasticsearch.cluster.ClusterStateTaskListener;
import org.elasticsearch.cluster.metadata.DataStream.TimestampField;
import org.elasticsearch.cluster.metadata.IndexAbstraction;
import org.elasticsearch.cluster.metadata.IndexMetadata;
import org.elasticsearch.cluster.metadata.IndexNameExpressionResolver;
import org.elasticsearch.cluster.metadata.IndexTemplateMetadata;
import org.elasticsearch.cluster.metadata.Metadata;
import org.elasticsearch.cluster.metadata.MetadataIndexTemplateService;
import org.elasticsearch.cluster.node.DiscoveryNode;
import org.elasticsearch.cluster.service.ClusterService;
import org.elasticsearch.common.Priority;
import org.elasticsearch.common.bytes.BytesReference;
import org.elasticsearch.common.regex.Regex;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.common.util.concurrent.AbstractRunnable;
import org.elasticsearch.common.xcontent.XContentHelper;
import org.elasticsearch.core.TimeValue;
import org.elasticsearch.core.Tuple;
import org.elasticsearch.env.Environment;
import org.elasticsearch.gateway.GatewayService;
import org.elasticsearch.index.IndexSettings;
import org.elasticsearch.index.VersionType;