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 (index < 0 || index >= list.size()) {
                        if (failOutOfRange) {
                            throw new IllegalArgumentException(
                                "["
                                    + index
                                    + "] is out of bounds for array with length ["
                                    + list.size()
                                    + "] as part of path ["
                                    + path
                                    + "]"
                            );
                        } else {
                            return false;
                        }
                    }
                    context = list.get(index);
                } catch (NumberFormatException e) {
                    return false;
                }
            } else {
                return false;
            }
        }
        String leafKey = fieldPath.pathElements[fieldPath.pathElements.length - 1];
        if (context instanceof Map<?, ?> map) {
            return map.containsKey(leafKey);
        }
        if (context instanceof List<?> list) {
            try {
                int index = Integer.parseInt(leafKey);