loading json file takes too long in rails 7

I have a JSON file with about a million entry, and when I load it in javascript like this:

<script>
var $select = $('#country');
$.getJSON("helpers/countries.json", function (data) {
  console.log(data.length);

  for (var i = 0; i < data.length; i++) {
   var value = data[i].name;
    var $option = $("<option/>").attr("value", value).text(value);
    $select.append($option);   
    }
});

</script>

It takes about 30 seconds to show the list of countries in the dropdown menu.

I have put the file in /public/helpers/countries.json on a server running ruby on rails.

I don’t want to use online APIs, I’d rather use local json file.