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
try {
                    index = Integer.parseInt(pathElement);
                } catch (NumberFormatException e) {
                    throw new IllegalArgumentException(
                        "[" + pathElement + "] is not an integer, cannot be used as an index as part of path [" + path + "]",
                        e
                    );
                }
                if (index < 0 || index >= list.size()) {
                    throw new IllegalArgumentException(
                        "[" + index + "] is out of bounds for array with length [" + list.size() + "] as part of path [" + path + "]"
                    );
                }
                context = list.get(index);
            } else {
                throw new IllegalArgumentException(
                    "cannot resolve ["
                        + pathElement
                        + "] from object of type ["
                        + context.getClass().getName()
                        + "] as part of path ["
                        + path
                        + "]"
                );
            }
        }
        String leafKey = fieldPath.pathElements[fieldPath.pathElements.length - 1];
        if (context == null) {
            throw new IllegalArgumentException("cannot set [" + leafKey + "] with null parent as part of path [" + path + "]");
        }