Yes, this has been discussed here many times.
When there are many rows in a repeater (this includes flex fields because they are just a special kind of repeater) and or there are a lot of fields for each layout, performance of the admin will degrade as more rows are added. Eventually this will cause a timeout on the saving of edits.
There are ways to improve initial page load, this can be done by setting fields like wysiwyg fields to not initialize until they are being edited.
There is nothing that can be done in ACf to improve/speed up saving under these conditions. ACF calls update_post_meta() or other similar function for each field. This means that every field creates its own DB queries to update the value. Repeaters do not scale well.
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