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
throw new IllegalArgumentException(
                    "[" + index + "] is out of bounds for array with length [" + list.size() + "] as part of path [" + path + "]"
                );
            }
            list.remove(index);
            return;
        }
        if (context == null) {
            throw new IllegalArgumentException("cannot remove [" + leafKey + "] from null as part of path [" + path + "]");
        }
        throw new IllegalArgumentException(
            "cannot remove [" + leafKey + "] from object of type [" + context.getClass().getName() + "] as part of path [" + path + "]"
        );
    }
    private static ResolveResult resolve(String pathElement, String fullPath, Object context) {
        if (context == null) {
            return ResolveResult.error("cannot resolve [" + pathElement + "] from null as part of path [" + fullPath + "]");
        }
        if (context instanceof Map<?, ?> map) {
            if (map.containsKey(pathElement)) {
                return ResolveResult.success(map.get(pathElement));
            }
            return ResolveResult.error("field [" + pathElement + "] not present as part of path [" + fullPath + "]");
        }
        if (context instanceof List<?> list) {
            int index;
            try {
                index = Integer.parseInt(pathElement);
            } catch (NumberFormatException e) {
                return ResolveResult.error(