Well, when dealing with AJAX, you can use the error log. Write anything you want to it and then look in the log.
http://php.net/manual/en/function.error-log.php
It’s a quick way to get information.
With what you say about using get_field() being slower, it could very well be that more queries are being created. While your testing you might want to look at the difference between using get_field with the field name vs. the field key.
This is all interesting
