I am not the developer, but I’ve been using ACf for a very long time.
In general, the reason that ACF stores all fields as separate entities is that ACF is only an admin tool and the ACF function are just wrappers for the build in WP function for meta and options values used for getting add updating these values. So, for example you can actually get the value of an ACF field by using get_post_meta();
The repeater fields, and all repeater type fields, this includes repeaters, flex fields, and groups, this was done to conform to above and at the same time to allow a way to associate all of the fields on the same row because WP does not provide any way to relate one meta field to another meta field.
The number of fields may affect front end performance slightly, however, it should not be significant. When WP sets up a post it automatically gets all of the meta values associated with the post in a single query.
How you code can effect performance, for example, if you do any coding that does a query on the database based on the meta value, this could effect performance because it causes WP to do a “LIKE” query on the meta value.
Where you will see performance drop off if you have a lot of fields is in the admin when making updates. This is due to the way that WP updates meta values as each field updated creates several queries. I personally do not concern myself with the performance of the admin.
