Over in #2003892-101: Convert date formats to config entities is was noted by @mradcliffe that:
There is a serious usability issue with the use of #ajax in this form that was introduced in #2003942: Convert system_configure_date_formats_form() to a controller and continued in the AjaxResponse. It is annoying that it does a request for each key stroke (keyup), which disrupts the normal flow of typing.