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
public void removeField(String path) {
        FieldPath fieldPath = new FieldPath(path);
        Object context = fieldPath.initialContext;
        for (int i = 0; i < fieldPath.pathElements.length - 1; i++) {
            ResolveResult result = resolve(fieldPath.pathElements[i], path, context);
            if (result.wasSuccessful) {
                context = result.resolvedObject;
            } else {
                throw new IllegalArgumentException(result.errorMessage);
            }
        }
        String leafKey = fieldPath.pathElements[fieldPath.pathElements.length - 1];
        if (context instanceof Map<?, ?> map) {
            if (map.containsKey(leafKey)) {
                map.remove(leafKey);
                return;
            }
            throw new IllegalArgumentException("field [" + leafKey + "] not present as part of path [" + path + "]");
        }
        if (context instanceof List<?> list) {
            int index;
            try {
                index = Integer.parseInt(leafKey);
            } catch (NumberFormatException e) {
                throw new IllegalArgumentException(
                    "[" + leafKey + "] is not an integer, cannot be used as an index as part of path [" + path + "]",
                    e
                );
            }
            if (index < 0 || index >= list.size()) {