If this really NEEDS to be a client-side-only solution, I still believe a worker is the only way to go. Only, in this case, it needs to behave like an API. So y

If this really NEEDS to be a client-side-only solution, I still believe a worker is the only way to go. Only, in this case, it needs to behave like an API. So your worker not only parses the JSON, but also responds to post messages with only the data that is requested from it. Why? Because a large JSON structure is most probably just a large JSON structure, but you most probably don't need it as a whole. You may need a total count of items, you may need a paginated set of items, or only a certain item or a set of fields of items — well, an API.