Unfortunately, no.
And by adding the load_value and save_post hooks you’ve slowed it down even more because ACF will still save the values to the database and load those values from the database. Using these hooks does not override the default ACF action.
It might be possible to do this if you use acf/save_post with a priority of < 10. This would run before ACF has saved the values. You could then loop over the repeater in $_POST[‘acf’] and create your JSON and then delete the repeater input from $_POST[‘acf’], deleting the input will prevent ACF from saving the values.
It’s probably too late to make a changes but 250 rows with 12 fields in each row tells me that this should have been done with a custom post type instead of a repeater.
