It is possible that we may need to implement a different strategy for large scale multisites like the one that you are hosting. Would you mind opening a new private support ticket via this link (https://www.advancedcustomfields.com/contact/) and ask for me, Elliot.
To determine why the page is taking so long to load, we will need to perform some debugging. If possible, please install the Query Monitor plugin and take a look at the Database Queries that are run during this 45s admin page.
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