classic reason why to add an openapi REST interface that has a paginated, cached index result list of pubkey -> user@example.com list
then you index it periodically and queries are cheap.
Discussion
also, i think this whole idea is dumb because nip-05s don't really prove anything, and queries for follow lists are already supported just fine
if the nip-05 name is needed, request it on demand of the display code that renders your graph or whatever way it's being done. there is also, other than that, the more useful point from the user's kind 0, and you can request them in batches easily