You are worrying about the wrong things… In your case your only worry should be the amount of time it will take to do database backups and restorations.
Now, reading from your description of how ACF is been used in your site, all you have to do, is to install a good cache plugin and things will always be fast, no matter how big your database gets, since while your pages are being served on the front end, the data will be coming from static pre-built html pages, and without the need to access the database (technically).
The issue with big databases that custom fields tend to create, is when (parts of) the data cannot be cached, for example, you have a car dealership website and you want to have an advanced, complex (parametric) search. An example to such search might be to list all cars located in London, are newer than the year 2015, and cost less than 20,000 pounds. In this case, the database has to be used to search for the records, and this is when custom fields will disappoint. And also, this is the exact reason why medium to large e-commerce sites that require such advanced searches will always opt for Magento, etc… over Woocommerce. There are 2 plugins (one free and one premium) that sort of mitigate this database issue, but neither of them support complex fields such as the repeater yet…
Anyway, in our case however, you will be just fine, have patience with your backups and install a good cache plugin.
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