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/IngestDocument.java
import org.elasticsearch.index.mapper.SourceFieldMapper;
import org.elasticsearch.index.mapper.VersionFieldMapper;
import org.elasticsearch.script.CtxMap;
import org.elasticsearch.script.TemplateScript;
import java.time.ZoneOffset;
import java.time.ZonedDateTime;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Base64;
import java.util.Collection;
import java.util.Collections;
import java.util.Date;
import java.util.HashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.Set;
import java.util.function.BiConsumer;
import java.util.stream.Collectors;
/**
 * Represents a single document being captured before indexing and holds the source and metadata (like id, type and index).
 */
public final class IngestDocument {
    public static final String INGEST_KEY = "_ingest";
    public static final String SOURCE_KEY = SourceFieldMapper.NAME; 
    private static final String INGEST_KEY_PREFIX = INGEST_KEY + ".";
    private static final String SOURCE_PREFIX = SOURCE_KEY + ".";
    public static final String PIPELINE_CYCLE_ERROR_MESSAGE = "Cycle detected for pipeline: ";
    static final String TIMESTAMP = "timestamp";