If this is a common occurrence the former would almost always be better, the second would be absolutely glacial. > but the best would be abstracting away order

If this is a common occurrence the former would almost always be better, the second would be absolutely glacial. > but the best would be abstracting away order fetching in application code to always fetch single orders and behind the scenes looking up the cache for singles and pooling all the misses into a single request to the plural endpoint. Unless a significant amount of requests have 100% cache hits then I doubt a local cache will make much of a difference at all, all it's saving is a bit of bandwidth.