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
if (string.isEmpty()) {
                    return;
                }
            }
        }
        setFieldValue(fieldPathTemplate.newInstance(templateModel).execute(), value, false);
    }
    private void setFieldValue(String path, Object value, boolean append) {
        setFieldValue(path, value, append, true);
    }
    private void setFieldValue(String path, Object value, boolean append, boolean allowDuplicates) {
        FieldPath fieldPath = new FieldPath(path);
        Object context = fieldPath.initialContext;
        for (int i = 0; i < fieldPath.pathElements.length - 1; i++) {
            String pathElement = fieldPath.pathElements[i];
            if (context == null) {
                throw new IllegalArgumentException("cannot resolve [" + pathElement + "] from null as part of path [" + path + "]");
            }
            if (context instanceof Map) {
                @SuppressWarnings("unchecked")
                Map<String, Object> map = (Map<String, Object>) context;
                if (map.containsKey(pathElement)) {
                    context = map.get(pathElement);
                } else {
                    HashMap<Object, Object> newMap = new HashMap<>();
                    map.put(pathElement, newMap);
                    context = newMap;
                }
            } else if (context instanceof List<?> list) {
                int index;