Too Long; Didn't Read
Enumerable data structures are meant for on-chain discovery and not front-end consumption. This means other protocols can easily and cheaply iterate over an account’s NFTs. However, at minting time, it becomes more and more expensive to loop over such items. If a user has, say 10 items, it will become quite expensive to check the list of ids and go through each one of the ids, consequently increasing the number of external calls from the protocol contract.