Hi guys
Thanks for this question.
Currently, the page_link and post_object fields load in all the choices in one go which is not a great idea for large sites as you have.
This has been a driving force behind the work on version 5. ACF v5 will use AJAX for all post_object and page_link fields to minimize the amount of data needed on the first page load!
v5 will be out in feb next year. Until then, I’m not sure what I can do to help, but wish you good luck with your project.
Thanks
E