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()) {
                    return true;
                } else {
                    if (failOutOfRange) {
                        throw new IllegalArgumentException(
                            "[" + index + "] is out of bounds for array with length [" + list.size() + "] as part of path [" + path + "]"
                        );
                    } else {
                        return false;
                    }
                }
            } catch (NumberFormatException e) {
                return false;
            }
        }
        return false;
    }
    /**
     * Removes the field identified by the provided path.
     * @param fieldPathTemplate Resolves to the path with dot-notation within the document
     * @throws IllegalArgumentException if the path is null, empty, invalid or if the field doesn't exist.
     */
    public void removeField(TemplateScript.Factory fieldPathTemplate) {
        removeField(renderTemplate(fieldPathTemplate));
    }
    /**
     * Removes the field identified by the provided path.
     * @param path the path of the field to be removed
     * @throws IllegalArgumentException if the path is null, empty, invalid or if the field doesn't exist.
     */