I don’t know this to be a fact, but I’m assuming that there is something built into ACF that does a simple “has something changes or not” test. If nothing has been changed in an ACF field then the data is just sent.
You can also create custom filters for what is allowed input in every field and I’m pretty sure this means that checking the data requires an AJAX call so that custom filters can be run and errors returned, so the larger the number of fields the more data that needs to be sent and returned during the request.
The answer to your question is that, all of the data is updated, but that’s not what’s slowing it down. Unfortunately I think this is just a side effect of the way that the work needs to get done.
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