Answers for "jquery show loading until page load"

on page fully loaded jquery

$(window).load(function () {
    ....
});

If you have to wait for an iframe (and do not care about the assets, just the 
DOM) - try this:

$(document).ready(function() { 
    $('iframe').load(function() { 
       // do something
    });
});

if page is loading then show loader in js

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.2.4/jquery.min.js"></script>