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
private final String[] pathElements;
        private final Object initialContext;
        private FieldPath(String path) {
            if (Strings.isEmpty(path)) {
                throw new IllegalArgumentException("path cannot be null nor empty");
            }
            String newPath;
            if (path.startsWith(INGEST_KEY_PREFIX)) {
                initialContext = ingestMetadata;
                newPath = path.substring(INGEST_KEY_PREFIX.length(), path.length());
            } else {
                initialContext = ctxMap;
                if (path.startsWith(SOURCE_PREFIX)) {
                    newPath = path.substring(SOURCE_PREFIX.length(), path.length());
                } else {
                    newPath = path;
                }
            }
            this.pathElements = newPath.split("\\.");
            if (pathElements.length == 1 && pathElements[0].isEmpty()) {
                throw new IllegalArgumentException("path [" + path + "] is not valid");
            }
        }
    }
    private static class ResolveResult {