Just guessing, but it looks like jsonlite relies on the uniqueness property of UUIDs to 'guarantee' uniqueness in what amounts to it's primary-key index, the in

Just guessing, but it looks like jsonlite relies on the uniqueness property of UUIDs to 'guarantee' uniqueness in what amounts to it's primary-key index, the index of ids. User-defined keys would need to be checked against all existing keys before inserting, to ensure uniqueness. Not impossible, but potentially much slower.