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/IngestService.java
static List<PipelineConfiguration> innerGetPipelines(IngestMetadata ingestMetadata, String... ids) {
        if (ingestMetadata == null) {
            return Collections.emptyList();
        }
        
        if (ids.length == 0) {
            return new ArrayList<>(ingestMetadata.getPipelines().values());
        }
        List<PipelineConfiguration> result = new ArrayList<>(ids.length);
        for (String id : ids) {
            if (Regex.isSimpleMatchPattern(id)) {
                for (Map.Entry<String, PipelineConfiguration> entry : ingestMetadata.getPipelines().entrySet()) {
                    if (Regex.simpleMatch(id, entry.getKey())) {
                        result.add(entry.getValue());
                    }
                }
            } else {
                PipelineConfiguration pipeline = ingestMetadata.getPipelines().get(id);
                if (pipeline != null) {
                    result.add(pipeline);
                }
            }
        }
        return result;
    }
    /**
     * Stores the specified pipeline definition in the request.
     */
    public void putPipeline(
        PutPipelineRequest request,