Most Efficient Way to Copy Values From Parent Object to Multi-Instance Child Object

Depending on your use case, it might be possible to use the JSON Splitter instead. It can split the child array into separate output documents, and copy any desired fields from the parent (using Include Paths) or all of the parent’s fields (using Include scalar parents). But I wouldn’t worry about a potential performance with Mapper unless you observe an actual performance issue with Mapper.

Hope that helps.